单项选择题

Clouds are formed from ______.

A．evaporation
B．water vapor
C．outer space
D．the hydrologic cycle
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单项选择题
This passage mainly discusses______.
A．water conservation
B．the hydrologic cycle
C．underground water
D．polluted ground water
单项选择题
The supply of ground water is getting low because of ______.
A．conservation
B．poisonous water
C．pollution
D．population increase
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