Answer:
The correct answer is: the third option: Scientists use seismic waves and how they move through the Earth.
Step-by-step explanation:
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Scientists have investigated the structure of the center of the earth, using earthquakes for it.
When an earthquake strikes, it produces many seismic waves. These waves are observed by scientists and recorded with their tools. From them they can know what is in the center of the earth and in each of its layers.
We conclude that the correct answer is: the third option: Scientists use seismic waves and how they move through the Earth.