Ambivalence means neutral opinions, which is incorrect in my opinion. The author does cautiously support the theory that the rock came from Mars, but is cautioning against making conclusions purely from this evidence. If the author thought the rock was irrelevant/did not make an opinion on its relevance, then I would think A is correct. Anyone else have any thoughts?

It is a very large subset of large wild species, but does not cover all large wild species. Like it says above, if humans have never seen Walrus as worth domesticating than they have never tried to domesticate Walrus. "A" improves the quality of the stimulus, but is not necessary

If you still are struggling with this, the correct answer closely conforms with the logic used in the stimulus. So you should be looking at the logical structure of the argument (eg. if a then b), and finding the answer that most closely conforms with this formatting.

I believe its because we are looking for an answer that could explain why the average beak size of wild birds decreased. It may not be a super strong answer, but its by far the best option available.