单项选择题

Sound waves reflected from a hill can be used to estimate ____________.

A．height of the hill
B．speed of sound
C．distance of the hill
D．intensity of sound
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单项选择题
The British used radio waves because they ____________.
A．were more exact than sound waves
B．could not be detected
C．could penetrate fog and clouds
D．were easier to use than sound waves
单项选择题
Radar enabled the English ____________.
A．detect German planes on their way to London
B．direct the outnumbered RAF planes effectively
C．confuse German bomber pilots
D．both A and B
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