The Smooth,dark Maria on the Moon Are

The smooth,dark maria on the Moon are:

A) immense impact basins that are smooth because they were covered by lava flows after the early, heavy bombardment had ended.
B) immense impact basins that are smooth because earlier craters were wiped out by the impact.
C) areas that were still molten at the time of the early, heavy bombardment and therefore show no evidence of the impacts.
D) regions that, although as old as the cratered highlands, escaped the early, heavy bombardment.