DESCRIPTION Title of Invention: CELLULOSE DERIVATIVE, RESIN COMPOSITION, MOLDED BODY, METHOD FOR PREPARATION THEREOF, AND CASE FOR ELECTRIC AND ELECTRONIC DEVICE Technical Field [0001] The present invention relates to a cellulose derivative, a resin composition, a molded body, a method for preparation thereof, and a case for electric and electronic devices. Background Art [0002] In members constituting electric and electronic devices such as a copy machine and a printer, various materials are used in consideration of characteristics and functions required for the members. For example, for a member (case) that plays a role of receiving a driving apparatus of an electric and electronic device or the like, and protecting the driving apparatus, . a large amount of PC (polycarbonate), an ABS (acrylonitrile-butadiene-styrene) resin, PC/ABS or the like are generally used (Patent Document 1). These resins are prepared by reacting compounds obtained by using petroleum as a raw material. [0003] By the way, fossil resources, such as petroleum, coal and natural gas, include carbon fixed under the earth over a long period of time as a main component. In the case where carbon dioxide is discharged into the atmosphere by combusting such fossil resources or products using the fossil resources as a raw material, carbon that does not exist in the atmosphere but is fixed deeply under the earth, is rapidly discharged as carbon dioxide, and thus, carbon dioxide in the atmosphere is largely increased, thereby causing a global warming. Accordingly, a polymer such as ABS and PC having petroleum, which is a fossil resource, as a raw material has excellent properties as a material of the member for electric and electronic devices, but since petroleum, which is a fossil resource, is used as the raw material, it is preferable that the amount used is decreased from the standpoint of preventing the global warming. [0004] Meanwhile, a plant-derived resin is basically generated by a photosynthesis reaction using water and carbon dioxide in the atmosphere as raw materials by plants. Therefore, there is an opinion that, although carbon dioxide is generated by combusting a plant-derived resin, the carbon dioxide corresponds to carbon dioxide previously existing in the atmosphere, and thus, the balance of carbon dioxide in the atmosphere becomes zero-sum, such that the total amount of CO2 in the atmosphere is not increased. Based on this opinion, the plant-derived resin is called a "carbon neutral" material. The use of the carbon neutral material instead of the petroleum-derived resin has gained importance in preventing the current global warming. [0005] Therefore, in the PC polymer, there is proposed a method for decreasing petroleum-derived resources by using plant-derived resources such as starch as a portion of the petroleum-derived raw materials (Patent Document 2). However, there is a need for further improvement from the standpoint of targeting a more perfect carbon neutral material. Related Art Patent Document [0006] Patent Document 1: Japanese Patent Application Laid-Open No. Sho 56-55425 Patent Document 2: Japanese Patent Application Laid-Open No. 2008-24919 Disclosure of Invention Problems to Be Solved by the Invention [0007] The present inventors paid an attention on using cellulose as a carbon neutral resin. Generally, however, cellulose does not have thermoplasticity, and therefore it is not appropriate for molding processing due to difficulty in molding by heating and the like. In addition, even if thermoplasticity could be imparted to cellulose, there still is the problem that strength such as impact resistance is largely deteriorated. There is also room for improvement on heat resistance. An object of the present invention is to provide a cellulose derivative and a resin composition that have good thermoplasticity, strength and heat resistance, and are suitable for molding processing. Means for Solving the Problems [0008] The present inventors found out, in consideration of a molecular structure of cellulose, that a cellulose derivative having a specific structure exhibits good thermoplasticity, impact resistance and heat resistance, thereby completing the present invention. That is, the above object can be accomplished by the following means. [1] A cellulose derivative, comprising A) a hydrocarbon group; B) a group containing an acyl group: -CO-RB and an ethyleneoxy group: -C2H4-O- (RB represents a hydrocarbon group); and C) an acyl group: -CO-Rc (Re represents a hydrocarbon group). [2] The cellulose derivative of [1] above, wherein A) the hydrocarbon group is an alkyl group having 1 to 4 carbon atoms. [3] The cellulose derivative of [1] above, wherein A) the hydrocarbon group is a methyl group or an ethyl group. [4] The cellulose derivative of any one of [1] to [3] above, wherein B) the group containing an acyl group: -CO-RB and an ethyleneoxy group: -C2H4-O- is a group containing a structure represented by the following Formula (1): Formula (1) [5] The cellulose derivative of any one of [1] to [4] above, wherein each of RB and Re independently represents an alkyl group or an aryl group. [6] The cellulose derivative of any one of [1] to [4] above, wherein each of RB and Re independently represents a methyl group, an ethyl group or a propyl group. [7] The cellulose derivative of any one of [1] to [6] above, wherein the cellulose derivative has substantially no carboxyl group. [8] A method for preparing the cellulose derivative of any one of [1] to [7] above, comprising a process of esterifying a cellulose ether containing a hydrocarbon group and a hydroxyethyl group: -C2H4-OH. [9] A resin composition comprising the cellulose derivative of any one of [1] to [8] above. [10] A case for electric and electronic device, composed of a molded body obtained by heating and molding the cellulose derivative of any one of [1] to [7] above or the resin composition of [9] above. [11] A method for preparing a molded body, comprising: a step of heating and molding the cellulose derivative of any one of [1] to [8] above or the resin composition of [9] above. Effects of the Invention [0009] The cellulose derivative or resin composition of the present invention has excellent thermoplasticity, and thus, may be manufactured into a molded body. Further, a molded body formed by the cellulose derivative or resin composition of the present invention has good impact resistance, heat resistance, and the like, and thus, may be used appropriately as component parts such as automobiles, home electric appliances, electric and electronic devices, mechanical parts, materials for housing and construction, and the like. In addition, the cellulose derivative or resin composition of the present invention is a plant-derived resin and is a material which may contribute to the prevention of global warming, and thus, the cellulose derivative or resin composition of the present invention may replace petroleum-derived resins of the related art. Furthermore, the cellulose derivative and resin composition of the present invention exhibit biodegradability, and thus, are expected to be used as a material with less environmental load. Embodiments for Carrying Out the Invention [0010] Hereinafter, the present invention will be described in detail. 1. Cellulose Derivative A cellulose derivative of the present invention contains: A) a hydrocarbon group, B) a group containing an acyl group: -CO-RB and an ethyleneoxy group: -C2H4-O- (RB represents a hydrocarbon group), and C) an acyl group: -CO-Rc (Re represents a hydrocarbon group). That is, the cellulose derivative in the present invention is obtained by substituting at least a portion of hydrogen atoms of a hydroxyl group contained in the cellulose {(CeHioOs),,} with A) the hydrocarbon group, B) the group containing an acyl group (-CO-RB) and an ethyleneoxy group (-C2H4-O-), and C) the acyl group (-CO-Rc). More specifically, the cellulose derivative in the present invention has a repeating unit represented by the following Formula (2). [0011] [Chem. 2] Formula (2) [0012] In the formula, each of R2, R3 and R6 independently represents a hydrogen atom, A) a hydrocarbon group, B) a group containing an acyl group (-CO-RB) and an ethyleneoxy group (-C2H4-O-), or C) an acyl group (-CO-Rc). Each of RB and Re independently represents a hydrocarbon group. However, at least a portion of R2, R3 and R6 represents a hydrocarbon group and at least a portion of R2, R3 and R6 represents a group containing an acyl group (-CO-RB) and an ethyleneoxy group (-C2H4-O-), and at least a portion of R2, R3 and R6 represents an acyl group (-CO-Rc). [0013] The cellulose derivative of the present invention is novel compounds, and may exhibit thermoplasticity because at least a portion of the hydroxyl groups of the P-glucose ring is etherified and esterified by A) the hydrocarbon group, B) the group containing an acyl group (-CO-RB) and an ethyleneoxy group (-C2H4-O-), and C) the acyl group (-CO-Rc), as described above, thereby being appropriate for molding processing. In addition, the cellulose derivative may exhibit excellent strength and heat resistance as a molded body, and particularly is useful as a thermo-molding material. Furthermore, since cellulose is a completely plant-derived component, cellulose is carbon neutral and may greatly reduce the environmental load. [0014] As used herein, "cellulose" means a polymer compound in which a plurality of glucose are linked by P-l,4-glycoside bonds with the hydroxyl groups bonded to the carbon atoms at the 2-, 3- and 6-position of each glucose ring of the cellulose being unsubstituted. Further, "hydroxyl groups contained in the cellulose" represents a hydroxyl group which is bonded to the carbon atoms at the 2-, 3- and 6-position of each glucose ring of the cellulose. [0015] The cellulose derivative in the present invention contains: at least one group in which a hydrogen atom of a hydroxyl group contained in the cellulose is substituted by A) a hydrocarbon group, at least one group in which a hydrogen atom of a hydroxyl group contained in the cellulose is substituted by B) a group containing an acyl group (-CO-RB) (RB represents a hydrocarbon group) and an ethyleneoxy group (-C2H4-O-), and at least one group in which a hydrogen atom of a hydroxyl group contained in the cellulose is substituted by C) a acyl group (-CO-Rc) (Re represents a hydrocarbon group). The cellulose derivative of the present invention may have two or more different kinds of groups as the A) to C). [0016] The cellulose derivative may contain A) the hydrocarbon group, B) the group containing an acyl group (-CO-RB) and an ethyleneoxy group (-C2H4-O-), and C) the acyl group (-CO-Rc) at any one part of the whole thereof, and may be composed of the same repeating unit and of a plurality of repeating units. In addition, it is not necessary for the cellulose derivative to contain all the substituents of the A) to C) in a single repeating unit. As more specific aspects, there may be the following aspects. (1) A cellulose derivative composed of a repeating unit in which a portion of R2, R3 and R6 is substituted with A) a hydrocarbon group, a repeating unit in which a portion of R2, R3 and R6 is substituted with B) a group containing an acyl group (-CO-RB) and an ethyleneoxy group (-C2H4-O-), and a repeating unit in which a portion of R2, R3 and R6 is substituted with C) an acyl group (-CO-Rc). (2) A cellulose derivative composed of the same repeating units in which any one of R2, R3 and R6 in a single repeating unit is substituted with A) a hydrocarbon group, B) a group containing an acyl group (-CO-RB) and an ethyleneoxy group (-C2H4-O-), and C) an acyl group (-CO-Rc) (that is, having all the substituents of A) to C) in a single repeating unit). (3) A cellulose derivative in which repeating units of different substitution positions or different kinds of the substituents in A) to C) are randomly linked. In addition, a part of the cellulose derivative may contain an unsubstituted repeating unit (that is, a repeating unit in which all of R2, R3 and R6 are a hydrogen atom in formula (1)). [0017] A) the hydrocarbon group may be any one of an aliphatic group and an aromatic group. When the hydrocarbon group is an aliphatic group, it may be straight, branched or cyclic and may have an unsaturated bond. Examples of the aliphatic group include an alkyl group, a cycloalkyl group, an alkenyl group, an alkynyl group and the like. Examples of the aromatic group may include a phenyl group, a naphthyl group, a phenanthryl group, an anthryl group and the like. A) the hydrocarbon group is preferably an aliphatic group, more preferably an alkyl group, and even more preferably an alkyl group having 1 to 4 carbon atoms (a lower alkyl group). Specific examples thereof include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a pentyl group, a hexyl group, a heptyl group, a 2-ethylhexyl group, a tert-butyl group, an isoheptyl group, and the like, with a methyl group or an ethyl group being preferred. [0018] In the acyl group (-CO-RB) in B), RB represents a hydrocarbon group. RB may be any one of an aliphatic group and an aromatic group. When RB is an aliphatic group, it may be straight, branched or cyclic and may have an unsaturated bond. The aliphatic group and aromatic group represented by RB may include the same as those described in A) the hydrocarbon group. RB may include preferably an alkyl group or an aryl group. The alkyl group or aryl group is preferably an alkyl group having 1 to 12 carbon atoms or an aryl group, more preferably an alkyl group having 1 to 12 carbon atoms, even more preferably an alkyl group having 1 to 4 carbon atoms, and most preferably an alkyl group having 1 or 2 carbon atoms (that is, a methyl group or an ethyl group). Specific examples of the RB include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a pentyl group, a hexyl group, a heptyl group, a 2-ethylhexyl group, a tert-butyl group, an isoheptyl group and the like. Preferably, RB is a methyl group, an ethyl group and a propyl group. [0019] B) the group containing an acyl group (-CO-RB) and an ethyleneoxy group (-C2H4-O-) is preferably a group containing a structure represented by the following formula (1). [0020] [Chem. 3] Formula (1) [0021] In the formula, RB represents a hydrocarbon group. The definition and preferable range of RB in formula (1) are the same as those previously described. The group of B) may include a plurality of ethyleneoxy groups, or only one group. More specifically, the group of B) may be represented by the following formula (1'). [0022] [Chem. 4] Formula (V) [0023] In the formula, RB represents a hydrocarbon group, n represents the repeating number, and is a number of 1 or more. The definition and preferable range of RB in Formula (T) are the same as those previously described. The upper limit of n is not particularly limited and changes depending on the amount of ethyleneoxy groups introduced. However, the upper limit is, for example, about 10. In the cellulose derivative of the present invention, a mixture of the group containing only one ethyleneoxy group in B) (a group in which n is 1 in formula (1')) and the group containing two or more ethylene groups in B) (a group in which n is 2 or more in formula (1')) may be contained. [0024] In C) the acyl group (-CO-Rc), Re represents a hydrocarbon group. As a hydrocarbon group represented by Re, one exemplified in the RB may be applied. The preferable range of Re is the same as that of the RB. [0025] In the cellulose derivative of the present invention, A) the hydrocarbon group, and the hydrocarbon group represented by RB and Re and ethylene group may have further substituents, or may be unsubstituted, but is preferably unsubstituted. In particular, when RB and Re have further substituents, it is preferred that a substituent which imparts water solubility, for example, a sulfonic acid group, a carboxyl group and the like, is not contained. A cellulose derivative which is water-insoluble and a molding material composed of the cellulose derivative may be obtained by excluding such a group. [0026] When A) the hydrocarbon group, RB, Re and the ethylene group have further substituents in the cellulose derivative of the present invention, examples of the substituent include a halogen atom (for example, a fluorine atom, a chlorine atom, a bromine atom and an iodine atom), a hydroxyl group, an alkoxy group (the number of carbons of an alkyl moiety is preferably 1 to 5), an alkenyl group and the like. Meanwhile, when A) the hydrocarbon group, RB or Re are other than an alkyl group, it is possible to have an alkyl group (preferably 1 to 5 carbon atoms) as a substituent. [0027] Further, when the cellulose derivative of the present invention is used as a molding material, the derivative is preferably water-insoluble. For this reason, it is preferred that the cellulose derivative has substantially no water-soluble substituent such as a carboxyl group, a sulfonic acid group and salts thereof. The cellulose derivative may have substantially no carboxyl group so as to be water-insoluble, thereby being more appropriate for molding processing. As used herein, "having substantially no carboxyl group" means to include the case where the cellulose derivative in the present invention has absolutely no carboxyl group, as well as the case where the cellulose derivative in the present invention has a very small amount of a carboxyl group in a range that the cellulose derivative is water-insoluble. For example, when a carboxyl group is contained in a raw material cellulose, a cellulose derivative in which the substituent of A) to C) is introduced by using the cellulose may contain a carboxyl group. However, the cellulose derivative is meant to be included in "a cellulose derivative having substantially no carboxyl group". In addition, "water-insoluble" means that the solubility in 100 parts by mass of water (pH 3 to 11) at 25°C is 5 parts by mass or less. The carboxyl group included in the cellulose derivative of the present invention is included in an amount of preferably 1% by mass or less and more preferably 0.5% by mass or less, based on the cellulose derivative. [0028] Specific examples of the cellulose derivative of the present invention include acetoxyethylmethylacetylcellulose, acetoxyethylethylacetyl cellulose, Acetoxyethylpropylacetyl cellulose, acetoxyethylbutylacetyl cellulose, Acetoxyethylpentylacetyl cellulose, acetoxyethylhexylacetyl cellulose, acetoxyethylcyclohexylacetyl cellulose, acetoxyethylphenylacetyl cellulose, acetoxyethylnaphthylacetyl cellulose, [0029] acetoxyethylmethylpropionyl cellulose, acetoxyethylethylpropionyl cellulose, acetoxyethylpropylpropionyl cellulose, acetoxyethylbutylpropionyl cellulose, acetoxyethylpentylpropionyl cellulose, acetoxyethylhexylpropionyl cellulose, acetoxyethylcyclohexylpropionyl cellulose, acetoxyethylphenylpropionyl cellulose, acetoxyethylnaphthylpropionyl cellulose, [0030] acetoxyethylmethylcellulose-2-ethylhexanoate, acetoxyethylethylcellulose-2-ethylhexanoate, acetoxyethylpropylcellulose-2-ethylhexanoate, acetoxyethylbutylcellulose-2-ethylhexanoate, acetoxyethylpentylcellulose-2-ethylhexanoate, acetoxyethylhexylcellulose-2-ethylhexanoate, acetoxyethylcyclohexylcellulose-2-ethylhexanoate, acetoxyethylphenylcellulose-2-ethylhexanoate, acetoxyethylnaphthylcellulose-2-ethylhexanoate, [0031] propionyloxyethylmethylacetyl cellulose, propionyloxyethylethylacetyl cellulose, propionyloxyethylpropylacetyl cellulose, propionyloxyethylbutylacetyl cellulose, propionyloxyethylpentylacetyl cellulose, propionyloxyethylhexylacetyl cellulose, propionyloxyethylcyclohexylacetyl cellulose, propionyloxyethylphenylacetyl cellulose, propionyloxyethylnaphthylacetyl cellulose, [0032] propionyloxyethylmethylpropionyl cellulose, propionyloxyethylethylpropionyl cellulose, propionyloxyethylpropylpropionyl cellulose, propionyloxyethylbutylpropionyl cellulose, propionyloxyethylpentylpropionyl cellulose, propionyloxyethylhexylpropionyl cellulose, propionyloxyethylcyclohexylpropionyl cellulose, propionyloxyethylphenylpropionyl cellulose, propionyloxyethylnaphthylpropionyl cellulose, [0033] propionyloxyethylmethylcellulose-2-ethylhexanoate, propionyloxyethylethylcellulose-2-ethylhexanoate, propionyloxyethylpropylcellulose-2-ethylhexanoate, propionyloxyethylbutylcellulose-2-ethylhexanoate, propionyloxyethylpentylcellulose-2-ethylhexanoate, propionyloxyethylhexylcellulose-2-ethylhexanoate, propionyloxyethylcyclohexylcellulose-2-ethylhexanoate, propionyloxyethylphenylcellulose-2-ethylhexanoate, propionyloxyethylnaphthylcellulose-2-ethylhexanoate and the like. [0034] The positions of substitution of A) the hydrocarbon group, B) the group containing an acyl group (-CO-RB) and an ethyleneoxy group (-C2H4-O-), and C) the acyl group (-CO-Rc) in the cellulose derivative and the numbers of each substituent per {3-glucose ring unit (the degrees of substitution) are not particularly limited. [0035] For example, the degree of substitution DSa of A) the hydrocarbon group (the number of A) the hydrocarbon groups with respect to the hydroxyl groups at the 2-, 3- and 6-positions of a P-glucose ring in a repeating unit) is preferably 1.0 45 g of hydroxyethylmethyl cellulose (trade name: Marpolose ME-250T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) and 2,250 mL of N,N-dimethyl acetamide were measured and put into a 5-liter three-necked flask equipped with a mechanical stirrer, a thermometer, a cooling tube and a dropping funnel, and stirred at room temperature. After it was confirmed that the reaction system became transparent and was completely dissolved, 129 mL of acetyl chloride was slowly added dropwise, followed by increasing the temperature of the system to 80°C to 90°C. Stirring was continued for 3 hr, and then the temperature of the reaction system was cooled to room temperature. The reaction solution was introduced into 10 L of water while vigorously stirring, whereupon a white solid was precipitated. The white solid was separated by suction filtration, and washed three times with large quantities of water. The obtained white solid was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-l) (acetoxyethylmethylacetyl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (57.8 g). [0097] 45 g of hydroxyethylmethyl cellulose (trade name: Marpolose ME-350T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) and 2,250 mL of N,N-dimethyl acetamide were measured and put into a 5-liter three-necked flask equipped with a mechanical stirrer, a thermometer, a cooling tube and a dropping funnel, and stirred at room temperature. After it was confirmed that the reaction system became transparent and was completely dissolved, 129 mL of acetyl chloride was slowly added dropwise, followed by increasing the temperature of the system to 80°C to 90°C. Stirring was contained for 3 hr, and then the temperature of the reaction system was cooled to room temperature. The reaction solution was introduced into 10 L of water while vigorously stirring, whereupon a white solid was precipitated. The white solid was separated by suction filtration, and washed three times with large quantities of water. The obtained white solid was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-2) (acetoxyethylmethylacetyl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (58.5 g). [0098] 45 g of hydroxyethylmethyl cellulose (trade name: Marpolose ME-350T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) and 2,250 mL of N,N-dimethyl acetamide were measured and put into a 5-liter three-necked flask equipped with a mechanical stirrer, a thermometer, a cooling tube and a dropping runnel, and stirred at room temperature. After it was confirmed that the reaction system became transparent and was completely dissolved, 158 mL of propionyl chloride was slowly added dropwise, followed by increasing the temperature of the system to 80°C to 90°C. Stirring was continued for 3 hr, and then the temperature of the reaction system was cooled to room temperature. The reaction solution was introduced into 10 L of water while vigorously stirring, whereupon a white solid was precipitated. The white solid was separated by suction filtration, and washed three times with large quantities of water. The obtained white solid was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-3) (propionyloxyethylmethylpropionyl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (79.2 g). [0099] 45 g of hydroxy ethylmethyl cellulose (trade name: Marpolose ME-350T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) and 2,250 mL of N,N-dimethyl acetamide were measured and put into a 5-liter three-necked flask equipped with a mechanical stirrer, a thermometer, a cooling tube and a dropping funnel, and stirred at room temperature. After it was confirmed that the reaction system became transparent and was completely dissolved, 93.1 mL of propionyl chloride was slowly added dropwise, followed by increasing the temperature of the system to 80°C to 90°C. Stirring was continued for 3 hr, the temperature of the reaction system was cooled to room temperature, and 100 ml of methanol and 500 ml of water were added. The reaction solution was introduced into 10 L of water while vigorously stirring, and a white solid was separated by suction filtration and washed three times with large quantities of water. The obtained white solid was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-4) (propionyloxyethylmethylpropionyl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (50.5 g). [0100] 45 g of hydroxyethylmethyl cellulose (trade name: Marpolose ME-350T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) and 2,250 mL of N,N-dimethyl acetamide were measured and put into a 5-liter three-necked flask equipped with a mechanical stirrer, a thermometer, a cooling tube and a dropping funnel and stirred at room temperature. After it was confirmed that the reaction system became transparent and was completely dissolved, 112.3 mL of butyryl chloride was slowly added dropwise, followed by increasing the temperature of the system to 80°C to 90°C. Stirring was continued for 3 hr, the temperature of the reaction system was cooled to room temperature, and 100 ml of methanol and 500 ml of water were added. The reaction solution was introduced into 10 L of water while vigorously stirring, and a white solid was separated by suction filtration and washed three times with large quantities of water. The obtained white solid was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-5) (butyryloxyethylmethylbutyryl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (55.2 g). [0101] 45 g of hydroxyethylmethyl cellulose (trade name: Marpolose ME-350T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) and 2,250 mL of N,N-dimethyl acetamide were measured and put into a 5-liter three-necked flask equipped with a mechanical stirrer, a thermometer, a cooling tube and a dropping funnel, and stirred at room temperature. After it was confirmed that the reaction system became transparent and was completely dissolved, a mixture solution of 38.7 mL of acetyl chloride and 46.6 mL of propionyl chloride was slowly added dropwise, followed by increasing the temperature of the system to 80°C to 90°C. Stirring was continued for 3 hr, the temperature of the reaction system was cooled to room temperature, and 100 ml of methanol and 500 ml of water were added. The reaction solution was introduced into 10 L of water while vigorously stirring, and a white solid was separated by suction filtration and washed three times with large quantities of water. The white solid was dissolved in methanol, and a white solid obtained by dropping the resulting solution into water was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-6) (acetoxyethylpropionyloxyethylmethylacetylpropionyl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (59.1 g). [0102] 45 g of hydroxyethylmethyl cellulose (trade name: Marpolose ME-350T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) and 2,250 mL of N,N-dimethyl acetamide were measured and put into a 5-liter three-necked flask equipped with a mechanical stirrer, a thermometer, a cooling tube and a dropping funnel, and stirred at room temperature. After it was confirmed that the reaction system became transparent and was completely dissolved, a mixture solution of 38.7 ml of acetyl chloride and 56.2 ml of butyryl chloride was slowly added dropwise, followed by increasing the temperature of the system to 80°C to 90°C. Stirring was continued for 3 hr, the temperature of the reaction system was cooled to room temperature, and 100 ml of methanol and 500 ml of water were added. The reaction solution was introduced into 10 L of water while vigorously stirring, and a white solid was separated by suction filtration and washed three times with large quantities of water. The white solid was dissolved in methanol, and a white solid obtained by dropping the resulting solution into water was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-7) (acetoxyethylbutyryloxyethylmethylacetylbutyryl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (60.2 g). [0103] 45 g of hydroxyethylmethyl cellulose (trade name: Marpolose ME-350T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) and 2,250 mL of N,N-dimethyl acetamide were measured and put into a 5-liter three-necked flask equipped with a mechanical stirrer, a thermometer, a cooling tube and a dropping funnel, and stirred at room temperature. After it was confirmed that the reaction system became transparent and was completely dissolved, a mixture solution of 46.6 mL of propionyl chloride and 56.2 mL of butyryl chloride was slowly added dropwise, followed by increasing the temperature of the system to 80°C to 90°C. Stirring was continued for 3 hr, the temperature of the reaction system was cooled to room temperature, and 100 ml of methanol and 500 ml of water were added. The reaction solution was introduced into 10 L of water while vigorously stirring, and a white solid was separated by suction filtration and washed three times with large quantities of water. The obtained white solid was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-8) (butyryloxyethylpropionyloxyethylmethylbutyrylpropionyl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (55.9 g). [0104] 30 g of hydroxyethylmethyl cellulose (trade name: Marpolose ME-250T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.), 0.74 g of methane sulfonic acid, and 57.4 ml of anhydrous acetic acid were measured and put into a 1-liter kneader (a biaxial Werner type kneader equipped with a sigma blade as a stirrer), and stirred at room temperature for 10 min, the temperature of the reaction system was increased to 35°C, and 120 ml of acetic acid was added dropwise over 30 min and maintained for another 2 hr to perform acetylation. 180 ml of water was slowly added dropwise while stirring. This doping solution was introduced into 420 ml of 10% diluted acetic acid while stirring, and then a white solid was precipitated. The white solid was separated by suction filtration, and washed with large quantities of water. The obtained white solid was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-9) (acetoxyethylmethylacetyl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (35.1 g). [0105] 30 g of hydroxyethylmethyl cellulose (trade name: Marpolose ME-250T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.), 0.37 g of sulfuric acid, and 57.4 ml of anhydrous acetic acid were measured and put into a 1-liter kneader (a biaxial Werner type kneader equipped with a sigma blade as a stirrer), and stirred at room temperature for 10 min, the .temperature of the reaction system was increased to 35°C, and 120 ml of acetic acid was added dropwise over 30 min and maintained for another 2 hr to perform acetylation. 180 ml of water was slowly added dropwise while stirring. This doping solution was introduced into 420 ml of 10% diluted acetic acid while stirring, and then a white solid was precipitated. The white solid was separated by suction filtration, and washed with large quantities of water. The obtained white solid was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-10) (acetoxyethylmethylacetyl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (36.9 g). [0106] 30 g of hydroxy ethylmethyl cellulose (trade name: Marpolose ME-250T; manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.), 0.74 g of methane sulfonic acid, and 77.2 ml of anhydrous propionic acid were measured and put into a 1-liter kneader (a biaxial Werner type kneader equipped with a sigma blade as a stirrer), and stirred at room temperature for 10 min, the temperature of the reaction system was increased to 35°C, and 120 ml of propionic acid was added dropwise over 30 min and maintained for another 2 hr to perform acetylation. 180 ml of water was slowly added dropwise while stirring. This doping solution was introduced into 420 ml of 10% diluted acetic acid while stirring, and then a white solid was precipitated. The white solid was separated by suction filtration, and washed with large quantities of water. The obtained white solid was dried under vacuum at 100°C for 6 hr to obtain a desired cellulose derivative (C-11) (propionyloxyethylmethylpropionyl cellulose; the degrees of substitution thereof are described in Table 1) as a white powder (37.6 g). [0107] Meanwhile, with respect to the compounds obtained in the above, the kinds of functional groups substituted with the hydroxyl groups (R2, R3 and R6) contained in cellulose, and DSa, MS, and DSb+DSc were observed and determined by 'H-NMR by using the method as described in Cellulose Communication 6, 73-79 (1999). [0108] The number average molecular weight (Mn) and weight average molecular weight (Mw) of the cellulose derivative obtained were measured. The measurement method thereof will be described below. [Molecular Weight and Molecular Weight Distribution] The number average molecular weight (Mn) and weight average molecular weight (Mw) were determined by using a gel permeation chromatography (GPC). Specifically, N-methylpyrrolidone was used as a solvent and a polystyrene gel was used, and the molecular weight was obtained by using a reduced molecular weight calibration curve previously prepared from a standard monodispersion polystyrene constitution curve. As the GPC device, HLC-8220GPC (manufactured by Tosoh Corp.) was used. [0109] The number average molecular weight (Mn), weight average molecular weight (Mw), and degree of substitution were incorporated and shown in Table 1. [0110] [0111] In Table 1, any of "B) a group containing an acyl group and an ethyleneoxy group" in the cellulose derivatives C-1, C-2, C-9 and C-10 is a group containing a structure of the following formula (1-1), any of "B) a group containing an acyl group and an ethyleneoxy group" in the cellulose derivatives C-3, C-4 and C-11 is a group containing a structure of the following formula (1-2), "B) a group containing an acyl group and an ethyleneoxy group" in the cellulose derivative C-5 is a group containing a structure of the following formula (1-3), "B) a group containing an acyl group and an ethyleneoxy group" in the cellulose derivative C-6 is a group containing a structure of the following formula (1-1) and a group containing a structure of formula (1-2), "B) a group containing an acyl group and an ethyleneoxy group" in the cellulose derivative C-7 is a group containing a structure of the following formula (1-1) and a group containing a structure of formula (1-3), and "B) a group containing an acyl group and an ethyleneoxy group" in the cellulose derivative C-8 is a group containing a structure of the following formula (1-2) and a group containing a structure of formula (1-3). [0112] [Chem. 5] [0113] The melting initiation temperatures of the obtained cellulose derivatives and Marpolose as a raw material were measured. The measurement method thereof will be described below. [Melting Initiation Temperature (Tm)] A melting initiation temperature was obtained by measuring the flow initiation temperature of a resin while increasing the temperature at a temperature increasing rate of 5°C/min with a load of 100 kg in a flow tester (manufactured by Shimadzu Corporation). The melting initiation temperature is shown in Table 2. [0114] The thermal decomposition initiation temperatures of the obtained cellulose derivatives, and Marpolose as a raw material were measured. The measurement method thereof will be described below. [Thermal Decomposition Initiation Temperature (Td)] A thermal decomposition initiation temperature was obtained by measuring a 2% weight reduction temperature of a sample while increasing the temperature at a rate of 10°C/min under nitrogen atmosphere by using a thermogravimetric/differential thermal analysis apparatus (manufactured by Seiko Instruments Inc.). The thermal decomposition initiation temperature is shown in Table 2. [0115] Table 2 [0116] As can be seen from Table 2, it is understood that the melting initiation temperature of the obtained cellulose derivative has been greatly reduced for Marpolose as a raw material. In addition, Td-Tm of the obtained cellulose derivative has been greatly increased for Marpolose as a raw material, which indicates that molding is easily carried out by using the thermoplasticity. [0117] The solubility of the obtained cellulose derivative and Marpolose as a raw material in water was measured. The measurement method of the solubility is as follows. [Measurement of solubility in water] Each sample was added to 100 g of water at 25°C and stirred to confirm whether the sample was dissolved. The results are shown in the following Table 3. Meanwhile, in the following Table 3, an amount of dissolution of 5 g or less is referred to as "insoluble" and an amount of dissolution of more than 5 g is referred to as "soluble". [0118] Table 3 [0119] In Table 3, H-l is ME-250T (Marpolose: manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.) and H-2 is ME-350T (Marpolose: manufactured by Matsumoto Yushi-Seiyaku Co.,Ltd.). From Table 3, it can be seen that hydroxyethylmethyl cellulose (H-l and H-2) is dissolved in water and the cellulose derivative of the present invention is insoluble. [0120] [Preparation of Test Piece] The cellulose derivative (C-l) obtained as described above was fed to an injection molding machine (manufactured by Imoto Machinery Co., Ltd., semi-automatic injection molding apparatus), and molded into a test piece for multipurposes (impact test piece and thermal deformation test piece) having a size of 4x10x80 mm at a cylinder temperature of 200°C, a mold temperature of 30°C, and an injection pressure of 1.5 kgf/cm2. [0121] In the same manner as in Example 1, test pieces were prepared by molding the cellulose derivatives (C-2) to (C-ll) and (H-l) ME-250T (Marpolose : manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.), (H-2) ME-350T (Marpolose : manufactured by Matsumoto Yushi-Seiyaku Co., Ltd.), (H-3) (manufactured by The Dow Chemical Company: ethyl cellulose; degree of substitution of ethoxy: 2.6), and (H-4) (manufactured by Eastman Chemical Company: Cellulose acetate propionate; degree of substitution of acetyl of 0.1; degree of substitution of propionyl: 2.5) as comparative compounds under the molding conditions of Table 4 to be described below. [0122] With respect to the obtained test pieces, the Charpy impact strength and heat deformation temperature (HDT) were measured by the following method. The results are shown in Table 4. [Charpy impact strength] In accordance with ISO 179, the test pieces molded by injection molding were provided with a notch having the front end of 0.25±0.05 mm and an incident angle of 45±0.5°, and stood still under the conditions of 23°C±2°C and 50%±5% RH for 48 hours or more, and then the impact strength was measured by a Charpy impact tester (manufactured by Toyo Seiki Seisaku-Sho, Ltd.) with the edge wise. [Heat DeformationTemperature (HDT)] In accordance with IS075, the temperature was measured when a predetermined bending load (1.8 MPa) was applied to the center of a test piece (in a flatwise direction), the temperature was increased at a constant rate, and thus a deformation of the center portion reaches 0.34 mm. [0123] Table 4 [0124] From the results in Table 4, it is understood that hydroxyethylmethyl celluloses (H-l) and (H-2) do not show thermoplasticity, while cellulose derivatives (C-l) to (C-ll) in Examples 1 to 11, which are modified with acyl groups in the celluloses, are given appropriate thermoplasticity, and thus are moldable, as well as exhibit high impact resistance and heat resistance. In addition, although (H-3) and (H-4) in Comparative Examples 3 and 4 were thermo-moldabie, it can be understood that cellulose derivatives (C-l) to (C-ll) in Examples 1 to 11 are moldable at low temperatures and equivalent or better results are imparted to cellulose derivatives (C-l) to (C-ll) in terms of Charpy impact strength and HDT, when compared to the (H-3) and (H-4). Industrial Applicability [0125] The cellulose derivative or resin composition of the present invention has excellent thermoplasticity, and thus may be manufactured into a molded body. Further, a molded body formed by the cellulose derivative or resin composition of the present invention has good impact resistance, heat resistance and the like, and thus may be used appropriately as component parts such as automobiles, home electric appliances, electric and electronic devices, mechanical parts, materials for housing and construction, and the like. In addition, the cellulose derivative is a plant-derived resin, and a material which may contribute to the prevention of global warming, and thus the cellulose derivative may replace petroleum-derived resins of the related art. Furthermore, the cellulose derivative and resin composition of the present invention exhibit biodegradability, and thus are expected to be used as a material with less environmental load. Although the present invention has been described with reference to detailed and specific embodiments thereof, it is obvious to those skilled in the art that various changes or modifications may be made without departing from the spirit and scope of the present invention. The present application claims priority from Japanese Patent Application No. 2009-187416 filed on August 12, 2009 and Japanese Patent Application No. 2009-295059 filed on December 25, 2009, the disclosure of which is incorporated herein by reference in its entirety. Claims [1] A cellulose derivative, comprising A) a hydrocarbon group; B) a group containing an acyl group: -CO-RB and an ethyleneoxy group: -C2H4-O- (RB represents a hydrocarbon group); and C) an acyl group: -CO-Rc (Re represents a hydrocarbon group). [2] The cellulose derivative of claim 1, wherein A) the hydrocarbon group is an alkyl group having 1 to 4 carbon atoms. [3] The cellulose derivative of claim 1, wherein A) the hydrocarbon group is a methyl group or an ethyl group. [4] The cellulose derivative of any one of claims 1 to 3, wherein B) the group containing an acyl group: -CO-RB and an ethyleneoxy group: -C2H4-O- is a group containing a structure represented by the following Formula (1): Formula (1) (wherein RB represents a hydrocarbon group.) [5] The cellulose derivative of any one of claims 1 to 4, wherein each of RB and Re independently represents an alkyl group or an aryl group. [6] The cellulose derivative of any one of claims 1 to 4, wherein each of RB and Re independently represents a methyl group, an ethyl group or a propyl group. [7] The cellulose derivative of any one of claims 1 to 6, wherein the cellulose derivative has substantially no carboxyl group. [8] A method for preparing the cellulose derivative of any one of claims 1 to 7, comprising a process of esterifying a cellulose ether containing a hydrocarbon group and a hydroxyethyl group: -C2H4-OH. [9] A resin composition comprising the cellulose derivative of any one of claims 1 to 8. [10] A case for electric and electronic device, composed of a molded body obtained by heating and molding the cellulose derivative of any one of claims 1 to 7 or the resin composition of claim 9. [11] A method for preparing a molded body, comprising: a step of heating and molding the cellulose derivative of any one of claims 1 to 8 or the resin composition of claim 9.