The Earth's Major Layers Formed Because_______

The earth's major layers formed because_______ .

A) the chemical composition of the solar nebula changed over time
B) the earth was heated after its formation resulting in melting and subsequent sinking of heavier materials
C) the earth's mantle grew like a crystal forming around the earth's core but the crust was exposed to space and stayed more like the original material
D) radioactive decay caused materials to change over time and resulted in the layering