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
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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
单项选择题
Without air, there would be no______.
A．shelters
B．volcanoes
C．weather
D．springs
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