The One Characteristic Shared by All Solitary White Dwarf Stars

The one characteristic shared by all solitary white dwarf stars is that they

A) have stopped generating thermonuclear energy but continue to shrink, thereby releasing gravitational energy as heat.
B) have never generated either thermonuclear or gravitational energy but are slowly cooling after their production in a supernova explosion.
C) are generating thermonuclear energy but are maintaining a constant radius and hence are not releasing gravitational energy.
D) have ceased to generate energy by thermonuclear processes or gravitational contraction and are slowly cooling down.