单项选择题

It is not widely known that______.

A．air is important to human being
B．air is important to the earth
C．air is necessary in many other ways
D．air is necessary to the animals
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单项选择题
The existence of electricity in the air has been known ______.
A．for hundreds .of years
B．for several years
C．recently
D．for thousands of years
单项选择题
Without air protection, the sun's rays______.
A．will become very cold
B．will do good to human beings
C．will become hot
D．will do harm to human beings
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