When we think of good health, we almost always think about eating good food and doing exercise. However, getting enough sleep an

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问题     When we think of good health, we almost always think about eating good food and doing exercise. However, getting enough sleep and dreaming are also part of good health. After a hectic day of work and play, the body needs to rest, and sleep is the body’s response to this need. Sleep is necessary for good health. During this time, the body recovers form the activities of the previous day. The rest that you get while sleeping enables your body to prepare itself for the next day.

    There are four levels of sleep, each being a little deeper than the one before. When you sleep, your muscles relax little by little. Your heart beats more slowly, and your brain slows down. After you reach the fourth level, your body shifts back and forth from one level of sleep to another.
    Although your mind slows down, from time to time you will dream. Scientists who study sleep say that when dreaming occurs, your eyeballs move more quickly (although your eyelids are closed). This stage of sleep is called REM, which stands for rapid eye movement. Not only all humans , but all mammals have been shown to experience this REM stage. Dreaming is a normal and necessary part of a good sleeping pattern, but no one knows why we dream.
    If you have trouble falling asleep, there are a few possible ways of solving this problem. Some people recommend breathing very slowly and very deeply. Other people believe that drinking warm milk will help make you drowsy. There is also an old suggestion that counting sheep will put you to sleep!
After you reach the fourth level of sleep, ______from one level to another.

选项 A、your muscles relax gradually
B、your body shifts back and forth
C、your heart beats more slowly
D、your brain slows down

答案B

解析 参见原文第二段最后一句。
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