Questions 31 to 35 are based on the following passage:

　　Judging from recent surveys, most experts in sleep behavior agree that there is virtually an epidemic (流行病) of sleepiness in the nation. “I can't think of a single study that hasn't found Americans getting less sleep than they ought to,”says Dr. David. Even people who think they are sleeping enough would probably be better off with more rest.

　　The beginning of our sleep-deficit (睡眠不足) crisis can be traced to the invention of the light bulb a century ago. From diary entries and other personal accounts from the 18th and 19th centuries, sleep scientists have reached the conclusion that the average person used to sleep about 9.5 hours a night. “The best sleep habits once were forced on us, when we had nothing to do in the evening down on the farm, and it was dark.” By the 1950s and 1960s, that sleep schedule had been reduced dramatically, to between 7.5 and eight hours, and most people had to wake to an alarm clock. “People cheat on their sleep, and they don't even realize they're doing it,” says Dr. David. “They think they're okey because they can get by on 6.5 hours, when they really need 7.5, eight or even more to feel ideally vigorous.”

　　Perhaps the most merciless robber of sleep, researchers say, is the complexity of the day. Whenever pressures from work, family, friends and community mount, many people consider sleep the least expensive item on his programme. “In our society, you're considered dynamic if you say you only need 5.5 hours' sleep. If you've got to get 8.5 hours, people think you lack drive and ambition.”��

　　To determine the consequences of sleep deficit, researchers have put subjects through a set of psychological and performance tests requiring them, for instance, to add columns of numbers or recall a passage read to them only minutes earlier. “We've found that if you're in sleep deficit, performance suffers,” says Dr.David. “Short-term memory is weakened, as are abilities to make decisions and to concentrate.”

　　31. People in the 18th and 19th centuries used to sleep about 9.5 hours a night because they had .

　　A) no electric lighting C) the best sleep habits

　　B) no drive and ambition D) nothing to do in the evening

　　32. According to Dr. David, Americans .

　　A) are ideally vigorous even under the pressure of life

　　B) can get by on 6.5 hours of sleep

　　C) do not know how to relax themselves properly

　　D) often neglect the consequences of sleep deficit

　　33. Many Americans believe that .

　　A) they need more sleep to cope with the complexities of everyday life

　　B) sleep is the first thing that can be sacrificed when one is busy

　　C) to sleep is something one can do at any time of the day

　　D) enough sleep promotes people's drive and ambition

　　34. The word “subjects”(Para. 4, Line 1) refers to .

　　A) the psychological consequences of sleep deficit

　　B) special branches of knowledge that are being studied

　　C) people whose behavior or reactions are being studied;

　　D) the psychological consequences of sleep definit

　　35. It can be concluded from the passage that one should sleep as many hours as is necessary to .

　　A) maintain one's daily schedule C) feel energetic and perform adequately

　　B) improve one's memory dramatically D) be considered dynamic by other people

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