单项选择题

When an airplane flies too high, ______.

A．the air will be too thin to support its wing
B．the air will become thicker
C．the air will exert pressure on it
D．the air will disappear in no time
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单项选择题
What would be ideal weather for flying a balloon
A．A hot day with no wind.
B．A cold, very windy day.
C．A clear day with a little wind.
D．A warm, rainy day.
填空题
the sun.
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