I was confused on C, but it makes sense now. Cats have to have different saliva and proteins because if they all had the same proteins then everyone who is allergic to any given set of saliva/protein would be allergic to all cats, because all cats would carry the saliva/protein that makes said person allergic.

BUT because it's common for cats to cause an allergic reaction to some but not all cat allergy people, we have to assume it's because not all cats have the same proteins.

I'm still confused on 6, D is wrong because we only know about - certain - variants, right?