Some time ago I did a homework in which I had to complete two-phrase dialogues by writing a meaningful response to a given question. No context was provided for the questions. One of the questions was,
За что тебе поставили четвёрку по сочинению?
За то, что я привела очень интересные факты и подробно обосновала свой вывод.
I got zero points for that question, but the teacher didn't mark any grammatical error of mine. He simply gave zero points. His style is to make students think and find own mistakes themselves.
But I'm at a loss as to what I could get wrong. The question seems to be very simple, "What did you get a B for the essay for?" I carefully checked my answer and see no grammatical error in it.
What's wrong with my answer?