单项选择题

Questions 15 to 17 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions.
Now listen to the passage.
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 serve.

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单项选择题

单项选择题

Questions 15 to 17 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage. What did P and S waves help scientists discover about the layers of the earth 您的浏览器不支持 audio 元素。