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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In which area may school play a more important role
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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