18．
Do you know why you sleep？You may think it's because you're tired．Or perhaps you've been told sleep is good for you．Yet no one knows exactly why or how people sleep．
Until the early 1950s，most scientists thought the body and the brain rested during sleep．Then a scientist watched people sleep．He noticed that during periods of between 5and 30minutes，sleepers'eyes made rapid movements，as if they were watching something．He called these periods REM （Rapid Eye Movement） sleep．He called periods when sleepers'eyes did not move non-REM sleep．
Whenever the scientist woke people during REM sleep，they were having vivid dreams．Measurements （测量） showed that sleepers'pulses （脉搏） went up quickly during REM sleep．Their breathing，heartbeat，and the blood going to the brain also increased．Far from being a time of rest，REM sleep is a time when the body is hard at work．Scientists still don't know the reason for this．Some believe REM sleep helps the brain grow．Others think REM sleep is a time for the brain to empty the day's garbage．
One thing all the scientists agree on is that sleep is necessary．Without enough sleep，people make mistakes．Most car accidents happen during the early morning hours when people are sleepiest．
66．During REM sleep，a person's breathing and heartbeatC．
A．stop B．sleep C．increase D．stay the same
67．The underlined word"vivid"meansD
A．easier B．large C．empty D．clear anti lively
68．D tells what happens when people don't get enough sleep．
A．Paragraph 1 B．Paragraph 2 C．Paragraph 3 D．Paragraph 4
69．The main idea of this passage isB．
．that REM sleep can help the brain grow B．about the scientific research of REM sleep
C．about how to empty the day's garbage D．that our body is good at work when sleeping
70．We can know from the passage thatA．
A．enough sleep is very important to people B．young people can experience REM sleep
C．the speed of sleepers'pulses will slow down during the REM sleep
D．all scientists have the same opinion on REM slerp．
