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
The word vista in the last line refers to _______.
A．support
B．prospect
C．history
D．visit
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