Why not D?

Hi Meredith, thanks for the question! The issue with answer choice D is that just because one piece of evidence (in this case, a single rock) was found to be from the Moon rather than Mars, does not mean that Mars was not affected by the late heavy bombardment. It only means that this single piece of evidence would not prove this. The falsity of one piece of evidence does not disprove a position. On the other hand, answer choice A does in fact basically rule out Mars being affected by the LHB. This would strengthen the theory that this event was experienced only by the Earth and the Moon. Hope this helps!

I can see why A is a better choice, but how would lack of evidence disprove a position? Thanks in advance

^^ I ruled this answer out bc the lack of evidence can't disprove a position (like in Logical Reasoning), right??

It certainly can't disprove, but our job is to weaken, not entirely disprove. While I agree that A is a weak weakener, it does slightly weaken the idea that the LHB occurred outside of earth and the moon by suggesting that it didn't happen on the nearest planet.