A is right since it is saying it did not occur during the early part of the moon's history. Early history = elastic = no change in gravitational properties. So it must have occurred later in moon's history when the surface is cold and hard (inelastic) so when a crater did form, the land below did not rise above; this will cause the high high negative gravity.
Hope explanation makes sense. I may have celebrated a little too much for 4th of July =/
