"Bread is food."
I wrote "빵이 음식입니다" and it was marked as wrong. But shouldn't it be equally as correct as "빵은 음식입니다"? Because when you write 빵은, you are saying "independent of other things, bread is food". But when you write 빵이, you are saying that, specifically, bread is food.
So they can both be correct in answer to different questions. And since we aren't given the preceding question, both should be accepted.