单项选择题

Airplanes usually do not need to climb quickly or fly high in the polar region because _______.

A．they do not land them
B．there are not clouds at all
C．they can cross the ice with special equipment
D．it is very cold
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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
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
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