单项选择题

What is the purpose of the talk

A．To review what students know about volcanic activity.
B．To demonstrate the use of a new measurement device.
C．To explain the answer to an examination question.
D．To provide background for the next reading assignment.

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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

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