单项选择题

Airplanes can climb quickly _______.

A．in warm air
B．in cold air
C．when it reaches the polar region
D．only when the clouds are down low
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单项选择题
The polar region is _______.
A．a good place to land at by airplane
B．a good place to fly over
C．a good place to travel to by land
D．a good place to live at
单项选择题
It is delight to fly at the Pole because there _______.
A．planes fly higher than at the equator
B．the eyes can reach about 4,000 meters
C．planes are clear of bad weather
D．planes fly more quickly than at the equator
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