Why Are Earth and Other Planets Spherical While Smaller Bodies

Why are Earth and other planets spherical while smaller bodies such as most asteroids are not?

A) Asteroids were never molten, and so never underwent differentiation.
B) The larger mass of a planet produces enough gravity to pull it into a round shape.
C) The atmosphere of a planet is heavy enough to push the planet into a round shape.
D) Earth and other planets formed from planetesimals pulled together by gravity into circular orbits.
E) It is the nature of massive objects to pull themselves into spherical shapes.