单项选择题

What does "it" of "since water-vapor becomes water when it cools" in Para. 3. refer to

A．Water-vapor.
B．Water.
C．Cloud.
D．Air.
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单项选择题
The third paragraph is concerned with ______.
A．how clouds rise in the air
B．how rain is formed
C．why water-vapor can float in the air
D．how clouds cause rain
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