This one is a bit tricky I think. Even though I knew the answer, some could see the clue as misleading, since a perfectly correct antonym of "before Christ" is "after Christ". It's trivial, but still correct as an antonym, and this is an antonym quiz. So there could be two possible correct answers, even though Anno Domini is the only correct one in the context of year counting. I would suggest that you maybe change the clue to BC? The answer would be AD, and it will be much less ambiguous that you are looking for the year counting answer. Alternatively, you could add a caveat in parentheses in that clue to indicate the specific meaning of that phrase that is required. You could also keep everything unchanged if you want to keep the quiz a bit tricky like that, which is entirely up to you ;)