单项选择题

It can be inferred from the statement about mother chimpanzees and their young (lines 21-23) that young chimpanzees have difficulty ______.

A．communicating with their mothers
B．adding quantities
C．making choices
D．opening hard nuts
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单项选择题
Why is the question of how easily people can get used to working at night not a mere academic question
A．Because few people like to reverse the cycle of sleep and wakefulness.
B．Because sleep normally coincides with the hours of darkness.
C．Because people are required to work at night in some fields of industry.
D．Because shift work in industry requires people to change their sleeping habits.
单项选择题
The best solution for implementing the 24-hour working system seems to be ______.
A．to change shifts at longer intervals
B．to have longer shifts
C．to arrange for some people to work on night shifts only
D．to create better living conditions for night workers
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