单项选择题

According to Dr. David, Americans ______.

A．are ideally vigorous even under the pressure of life
B．often neglect the consequences of sleep deficit
C．do not know how to relax themselves properly
D．can get by on 6.5 hours of sleep

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单项选择题

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

A．improve one's memory dramatically
B．be considered dynamic by other people
C．maintain one's daily schedule
D．feel energetic and perform adequately

单项选择题

The author's purpose in presenting the first paragraph is ______.

A．to explain the background and new features of today's astronomy
B．to discuss in detail the most recent achievements in space research
C．to introduce some newly established space stations
D．to illustrate that the world today is different in many aspects from that of a generation ago