Why Do the Maria Have Few Craters in Them

Why do the maria have few craters in them?

A) The maria are ancient sea beds. During the heaviest bombardment they were filled with water, and the meteoroids were broken up by the water, doing little damage to the maria themselves.
B) The maria are surfaced with hard basalt. Impacts mostly bounce off or break apart without making a crater.
C) The basalt that fills the maria has a low melting temperature. An impact will melt it, and it flows immediately to obliterate the crater.
D) There has been relatively little cratering activity since the maria were formed.