单项选择题

Practical application of this principle resulted in ______.

A．new electronic instruments for planes
B．new radio sets for RAF bombers
C．an electronic detecting device
D．a radio directing and ranging device
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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
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
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