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 temperature.
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单项选择题
What is one reason astronomers make long-time exposures
A．To spend less time at their telescopes.
B．To overcome the problem of weak light.
C．To take more photographs.
D．To photograph astronomical objects without using a telescope.
单项选择题
The shuttle flies to the earth at ______ an hour after reentering the atmosphere.
A．7,000
B．17,000
C．27,000
D．70,000
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