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
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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 is the main topic of this speech
A．The growing crisis of inadequate retirement pensions for the future.
B．The problem of providing adequate medical care for the elderly.
C．The need to reevaluate our attitudes towards senior citizens.
D．The need to reform the present pension system.
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