单项选择题

Directions: There are 20 blanks in the following passage. For each blank there are four choices marked A), B), C) and D) on the right side of the paper. You should choose the ONE that best fits into the passage. Then mark the corresponding letter on the Answer Sheet 2 with a single line through the centre.
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 1 less sleep than they ought to," says Dr. David. Even people who think they are sleeping enough would probably be better off 2 more rest.
The beginning of our sleep-deficit (睡眠不足) crisis can be 3 to the invention of the light bulb a century ago. From diary entries and other personal 4 from the 18^th and 19^th centuries, sleep scientists have 5 that the average person used to sleep 9.5 hours a night. "The best sleep habits once were forced on us, 6 we had nothing to do in the evening down on the farm, and it was dark." By the 1950s and 1960s, that sleep schedule 7 dramatically, to between 7.5 and 8 hours, and 8 people had to wake to an alarm clock. "People cheat on their sleep, 9 they don’t even realize they’re doing it," says Dr. David, " They think they’re okay because they can 10 on 6.5 hours, when they really need 7.5, 8 or even more to feel 11 vigorous.
Perhaps the most merciless robber of sleep, researchers say, is the complexity of the day. Whenever 12 from work, family, friends and community mount (上升), many people consider sleep the least 13 item on his program. "In our society, you 14 dynamic if you say you only need 5.5 hours’ sleep. If you’ve got to get 8.5 hours, people think you 15 drive and ambition. "
To determine the 16 of sleep deficit, researchers have put subjects through a set of psychological and performance tests 17 them, for instance, to add columns of numbers or 18 a passage read to them only minutes earlier. "We’ve found that if you’re in sleep deficit, performance 19 ," says Dr. David, " Short-term memory is weakened, 20 are abilities to make decisions and to concentrate. "