单项选择题

There is no living creature that does not need sleep or complete rest every day. If you want to know why, just try (41) without sleep for a long period of time! You (42) discover that your mind and body would become (43) tired to work properly. You would become irritable and find it hard to think clearly or (44) on your work. So sleep is quite (45) the time when the cells of your body (46) from the work of the day and (47) supplies of energy for the next (48) of activity.
One of the things we all know about sleep is that we are (49) in sleep. We do not know what is (50) on around us. But that does not mean the body stops (51) activity. The vital organs continue to work during sleep, but most of the body functions are (52) down.
For example, our breathing becomes slower and deeper. The heart beats (53) slowly, and the blood pressure is lower. Our arms and legs become limp(软弱的), and muscles that control our posture(姿势) are (54) . It would be impossible for our body to relax to such an (55) if we were awake. So sleep does for us (56) the most quiet rest cannot do.
Your body temperature becomes lower when you are asleep, (57) is the reason people go to sleep under some kind of (58) . And even though you are unconscious, ninny of your reflexes (反射作用) still work. (59) , if some one tickles your foot, you will pull it away in your sleep, or even (60) a fly from your forehead. You do these things without knowing it.

54（）

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