2. 外语
  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.
When a person is seen dozing, it suggests that he ______.
选项 A、is sleepy
B、is awake
C、is falling asleep
D、has fallen asleep
答案D
解析 第二段的开头讲到睡眠的第一阶段，“Some people call it dozing.”说明打盹的人也是睡着了的。所以正确答案是D。选项C表示“马上要睡着”，因此是不正确的。
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