Final answer:
Seismic waves, generated by earthquakes, are a key indirect evidence for understanding the Earth's internal structure as their behavior changes when traveling through different materials inside the Earth.
Step-by-step explanation:
The indirect evidence that shows what the center of the Earth is composed of includes seismic waves (option A). These waves are generated by events like earthquakes and can provide insight into the Earth's interior structure. As seismic waves travel through the Earth, they are affected by the composition and state of the Earth's internal materials, changing speed and direction as they move through layers of varying densities. This behavior allows scientists to infer details about these layers, distinguishing between solid and liquid regions, and ultimately helping construct a model of the Earth's core and mantle.
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