The Caloris Basin on Mercury Covers a Large Region of the Planet

The Caloris Basin on Mercury covers a large region of the planet, but few smaller craters have formed on top of it. From this we conclude that

A) the Caloris Basin formed toward the end of the solar system's period of heavy bombardment.
B) the Caloris Basin was formed by a volcano.
C) Mercury's atmosphere prevented smaller objects from hitting the surface.
D) erosion destroyed the smaller craters that formed on the basin.
E) only very large impactors hit Mercury's surface in the past.