Scientists Now Understand That the Earth Consists of Layers, with the Densest

Scientists now understand that the Earth consists of layers, with the densest materials in the core. What allowed the differentiation of the Earth's layers to happen?

A) a giant rock from space hit the Earth and made a huge hole almost to the center, through which material could move
B) continental drift and plate tectonics made this happen
C) the early Earth must have been so hot it was like a liquid and heavier things sank to the middle
D) this is one of the things that happened in the last few thousand years because of global warming
E) this is an unsolved problem; scientists have no good ideas about this at all