题目内容：
Different Stages of SleepA scientist who has spent many years studying sleep was asked this question: "Why do we need sleep?" His answer was."If we do not sleep, we get very sleepy, "No one knows exactly why we sleep．But scientists have learned a lot．From．sleep experiments, scientists have [earned that there are four different stages 0f 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 this kind of sleep dozing (打盹)．Scientists call it Stage 1 sleep.
Next, if you are no,t awakened, you drift into a deeper sleep．Your pu!se and breathing become even slower than they were during Stage 1 sleep．But you can still be awakened quite easily．If you take a catnap (小睡，打盹), you probably will not get any further than this stage of sleep, which is called Stage 2.
If you sleep longer than about twenty minutes, you go into the third stage of sleep.Your body is very relaxed．It would take a loud noise to wake you up.
You have probably heard of people who walk in their sleep．No one knows what
makes people sleepwalk (梦游)．But a person can sleepwalk only during Stage 4．This is the deepest kind of Sleep．I{ someone wakes you up during Stage 4, you might, feel very confused．It might take you a few minutes to get used to being awake.
After about ten minutes of Stage 4 sleep, you go back to Stage 3 and then to Stage 2.Then, something quite different begins.to happen．Your heartbeat becomes rapid．Your eyelids flutter (跳动), and your eyes move．This stage of sleep is called REM (rapid eye movement) sleep.
Most dreams happen during REM sleep． Scientists who study sleep often make noisesor talk to sleeping people in the REM stage．They are trying to find out what effects noises can have on dreams． As sleep research continues, scientists might also learn what causes dreams and how dreams might be helpful to people.According to the author, why do we need sleep? A．If we do not sleep, we get very sleep
B．Because there are four different stages of slee
C．No one knows the caus
D．It is not mentioned in the articl
参考答案：
答案解析：