How Does the Differentiation Process Generate Heat

How does the differentiation process generate heat?

A) As dense material sinks through less dense rock, it generates heat through friction.
B) As low-density rock rises to the surface, it carries heat from the interior of the planet.
C) Heat is generated by the constant bombardment of the planet by planetesimals.
D) Iron and nickel accumulating at the center of the planet concentrate the planet's heat.