How Is It That White Dwarfs, Which Are Relatively Small

How is it that white dwarfs, which are relatively small, can have surface temperatures of several tens of thousands of kelvins?

A) They are actually very similar in size and temperature to small main-sequence stars like red dwarfs.
B) The nuclear fusion in the core of a white dwarf involves carbon and oxygen, and this produces more energy and heat than either hydrogen or helium fusion.
C) The surface of the white dwarf is actually the core since the cooler outer layers have been blown off.
D) Because white dwarfs are fully convective, the nuclear fusion takes place on the surface.