Detailed Instructions: You are given a science question (easy-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.
Q: Earth's mantle, primarily composed of silicate and magnesium oxide rocks, floats on top of the liquid iron and nickel outer core. Which difference between these two layers of Earth resulted in their differentiation?
(A) age (B) velocity (C) viscosity (D) density
A:
D