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  3. No one knows exactly why we sleep. But scientists have learned a lot about how we sleep. When you first fall asleep, your hea

No one knows exactly why we sleep. But scientists have learned a lot about how we sleep. When you first fall asleep, your hea

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问题    No one knows exactly why we sleep. But scientists have learned a lot about how we sleep.
   When you first fall asleep, your heart starts to beat more slowly and your breathing slows down. If something wakes you, you might not think that you had been asleep. Some people call it dozing (打盹). Next, you enter into a deeper sleep. Your pulse (脉搏) and breathing become even slower. But you can still be awakened quite easily. If you take a catnap, you probably will not get any further than this stage (阶段) of sleep. If you sleep longer than about twenty minutes, you get into the third one. Your body is very relaxed. It would take a loud noise to wake you up. You may have heard of people who walk in their sleep. No one knows what makes people sleepwalk. But a person can sleepwalk only during stage four. This is the last and deepest kind of sleep. If someone wakes you up, you might feel very uncomfortable and you need a few minutes to get used to being awake. After about ten minutes of this stage sleep, you go back to stage three and then to stage two. Then something quite different begins to happen. Your heartbeat becomes rapid. Your eyelids flutter (颤动), and your eyes move. This stage of sleep is called REM (rapid eye movement) sleep.
   Most dreams happen during REM sleep. Scientists who study dreams often make noises or talk to sleeping people in the REM stage. They are trying to find out what effects noises can have on dreams, what causes dreams and how dreams might be helpful to people.
The sleep experiment show that there are ______ different stages of sleep.
选项 A、two
B、three
C、four
D、five
答案C
解析 注意第二段中一些表明顺序的词和短语．如：“first…，Next…，you get into the third one...But a person can sleepwalk only during stage four. This is the last and deepest kind…”。这些词和短语明显地表明：睡眠一共有4个阶段。因此正确答案是C。
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