The process of the gaining or losing weight can be explained by comparing your body to you

r car. Both run 56 fuel，food for your body and gasoline for your car. Both 57 that fuel，first into heat，then energy，some of 58 is used to do work，and some emitted as waste. And 59 your car uses more energy when the engine is racing than when it is idling，

60 does your body use more energy when you are working hard than 61 you are resting.

For the purpose of this comparison， 62 ，there is one significant difference between them. Your car cannot store fuel by turning it into 63 else；all gasoline not 64 remains as gasoline. But your body stores 65 energy as fat. When the gas tank is 66 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 67 of two things：eat more calories（units of heat，therefore energy），or use less through 68 . If you want to lose weight，you do the 69 ：decrease your intake of calories or increase the amount of energy you spend. There is 70 way gaining or losing weight is always a relation between intake and output of potential energy.

56_______

A. in

B. on

C. for

D. against