关于配合，下列说法中正礁的是（）。A、即使间隙等于零，该配合

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问题单项选择题

关于配合，下列说法中正礁的是（）。

A、即使间隙等于零，该配合也是间隙配合

B、过盈配合中轴的基本尺寸大于孔的基本尺寸

C、间隙配合中孔的公差带在轴的公差带之下

D、过渡配合中孔的公差带和轴的公差带相互交叠

答案

参考答案：D

阅读理解

Judging from recent surveys, most experts in sleep behaviour agree that there is 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-lack crises can be traced back to the invention of the light bulb a century

ago. From diary entries and our 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 on the farm, and it was dark.” By the 1950s and 1960s, the sleep schedule had been reduced dramatically, to between 7.5 and 8 hours, and most people had to wake to an alarm clock. “People cheat in their sleep, and they don’t even realize

they’re doing it,” says Dr. David. “They think they’re okay because they can manage with 6.5 hours, when they really need 7.5, 8 or even more to feel ideally energetic.”

Perhaps the most cruel robber of sleep, researches say, is the complexity of the day. Whenever

pressures from work, family, friends and community increase, many people consider sleep the least

expensive item on their programme. “In our society, you’re considered energetic if you say you need only 5.5 hours’ sleep. If you have to get 8.5 hours, people think you lack drive and ambition.”

To determine the consequences of sleep-lack, 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 lacking in sleep, performance

suffers,” says Dr. David. “Short-term memory is weakened, so are abilities to make decisions and to

concentrate.”

1. What is the main topic of the passage?

A. Research on the causes and consequences of sleep-lack.

B. The epidemic of sleepiness in the modern times.

C. The history of people’s sleeping patterns.

D. The minimum of our sleeping hours.

2. Which of the following is Dr. David’s opinion?

A. People who think they are sleeping enough are better off than those who don’t.