单项选择题

What did P and S waves help scientists discover about the layers of the earth

A．How deep they are.
B．Where earthquakes form.
C．How hot they are.
D．What purpose they serv
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单项选择题
What important discovery about seismic waves does the instructor mention
A．They occur at regular intervals.
B．They can withstand great heat.
C．They travel through the Earth's interior.
D．They can record the Earth's internal temperatur
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What did the man think about the people of the Ice Age
A．They lived in caves.
B．They traveled in groups.
C．They had an advanced language.
D．They ate mostly fruit.
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