Why does the correct answer weaken?

I understand how it provides an alternative explanation; it might just be a bear walking normally and not cross-walking humans but what if the bears weren't even at the site? Doesn't that still leave open the possibility that it was cross-walking humans? I also don't get why C is wrong. Another explanation states that this choice was wrong because maybe the humans decided to cross-walk in one area of the site then walk normally in site M, but both this choice and the correct one have these uncertainties so how do I know which one to pick?

I also don't get why D is incorrect. I saw another explanation but I'm still at a loss. If the footprints the two paleontologists were looking at were incomplete, couldn't that also cast doubt that they were human footprints?