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
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
单项选择题
What do you think the speaker will talk about in the next part of his speech
A．The contributions the elderly can make to our societies.
B．The swelling burden of supporting welfare programs in the future.
C．Our responsibility of building retirement homes for our graying society.
D．Our responsibility of looking after the old family members by ourselves.
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