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
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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