单项选择题

What is the main topic of the passage

A．The mechanics of rain.
B．The climate of North America.
C．How gravity affects air current.
D．Types of clouds.
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单项选择题
In this passage, what does the term coalescence refer to
A．The gathering of small clouds to form large clouds.
B．1'he growth of droplets.
C．The fall of raindrops and other precipitation.
D．The movement of dust particles in the sunlight.
单项选择题
Why don't all ice crystals in clouds immediately fall to Earth
A．They are balanced by the pressure of rain droplets.
B．The effect of gravity at high altitude is random.
C．They are kept in the sky by air current.
D．They are moving about with fixed direction.
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