题目内容：
回答题。The Process of Gaining or Losing Weight
The process of gaining or losing weight can be explained by comparing your body to your car. Both run 51 fuel, food for your body and gasoline for your car. Both 52 that fuel, first into heat, then energy, some of 53 is used to do work, and some emitted as waste. And 54 .your car uses more energy when the engine is racing than when it is idling, 55 does your body use more energy when you are working hard than 56 you are resting.
For the purpose of this comparison, 57 , there is one significant difference between them. Your car cannot store fuel by turning it into 58 else ; all gasoline not 59 remains as gasoline. But your body stores 60 energy as fat. When the gas tank is 61 empty, the car won't run ; but your body can burn fat to provide more energy.
Therefore, if you want to gain weight, you must do 62 of two things : eat more calories ( units of heat, therefore energy ) , or use less through 63 . If you want to lose weight, you do the 64 , decrease your intake of calories or increase the amount of energy you spend. There is 65 way. Gaining or losing weight is always a relation between intake and output of potential energy.
A．in
B．on
C．for
D．against
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