单项选择题

This passage mainly discusses______.

A．water conservation
B．the hydrologic cycle
C．underground water
D．polluted ground water
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单项选择题
What percentage of the total earth's water does underground water make up
A．0.05%.
B．0.5%.
C．4.5%.
D．5%.
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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