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
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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