心肌细胞的自律性以窦房结最高，正常为（）

A.25～40／min

B.10～25／min

C.40～60／min

D.80～120／min

E.60～100／min

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 completely empty, the car won’t run; but your body can burn fat to provide more energy.

58().

A.everything 

B.nothing 

C.something 

D.anything