单项选择题

It can be inferred from this passage that ______.

A．the rain is caused by water-vapor
B．the higher the air rises, the higher the temperature is
C．the higher the air rises, the lower the temperature is
D．the clouds rise, and the water drops become smaller

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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.

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