单项选择题

The estimate is based on the______.

A．direction of the hill
B．accepted speed of sound
C．interval between sound and echo
D．both B and C
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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
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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