Why Are Almost All the Craters We See on the Moon

Why are almost all the craters we see on the Moon circular?

A) because the rocks and icebergs that fell from space to make them were all spherical
B) because the volcanoes that make them erupt in a symmetrical way, making circular openings for the lava to emerge
C) because the active surface of the Moon converts the craters into circular basins, no matter what shape they start out being
D) because the rocks and icebergs that hit the Moon are moving so fast, they explode on impact, making a circular hole
E) this is an unsolved mystery; it's one of the most puzzling things about the Moon