discussions among earth scientists about new data scientific books defining the layers of Earth a lead scientist explaining the structure of Earth a physical model of Earth showing its layers
I gave my students this multiple choice question: Some scientists once believed that Earth was solid like a marble. Which of these has been most responsible for changes in the scientific view of the structure of Earth?

Only one answer is correct among these 4 choices:
- discussions among earth scientists about new data
- scientific books defining the layers of Earth
- a lead scientist explaining the structure of Earth
- a physical model of Earth showing its layers

Could you tell me which one is correct?

discussions among earth scientists about new data