单项选择题

The practical use of radio waves brought about______.

A．new machines for planes
B．a radio directing and ranging device
C．new radio sets for RAF bombers
D．a discovering device
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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 discovered
C．could pass through fog and clouds
D．were easier to use than sound waves
单项选择题
Sound waves reflected from a hill can be used to calculate the______.
A．Speed of sound
B．time between a sound and ifs echo
C．height of the hill
D．distance of the hill
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