It depends on the style manual you are following. The more conservative use is "A's and B's". The apostrophe does not show possession in this case.
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Anonymousyou'd think from a novelist of McEwan's calibre and from a booker-award winning novel it would have perfect grammar.Good novelists don't necessarily follow the rules of prescriptive grammarians.
fivejedjonGood novelists don't necessarily follow the rules of prescriptive grammarians.So true. That's why I give them A's.