Why would there be two ways to classify the layers of the earth?

Which sentence best expresses the differing opinions of these two book reviews?

Which of the following statements is true? 1. the mantle of the earth is made of solid nickel and iron. 2. the crust of the earth is made of solid nickel and iron. 3. the inner core is made of solid nickel and iron.

What is the name of the two satelites on mars