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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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.
单项选择题
Where does this speech most likely take place
A．At a retirement home.
B．At a conference center.
C．At a hospital.
D．In a school class.
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