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
A．exactB．someC．largeD．enough
A．exact
B．some
C．large
D．enough
单项选择题
Without air, there would be no______.
A．shelters
B．volcanoes
C．weather
D．springs
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