The Earth’s interior is composed of several layers: the crust, the mantle and the core. Scientists used waves and their speed to study the Earth's interior. The speed of seismic waves is affected by the properties of the material the waves pass through. The stiffness of material affects the speed of these waves. P-waves, or pressure waves, go through both liquids and solids, while S-waves, or shear waves go through solids but not liquids. The following helped scientists understand that Earth's interior is liquid: S waves cannot travel through liquids. Correct answer: D
