Could the teacher have just made a mistake? It seems possible if she was correcting a lot of papers.
Last year, we had a rigid teacher who marked a correct answer wrong. I put a post-it note on the test paper saying that we tried to rework the solution at home and did not know how else to do it other than the way DS had done it on the test. That evening, the difficult and rigid teacher was waiting for me at the pick up line and profusely apologized for the mistake that she had made and said that she had updated the grades correctly for DS. I expected a lot of abrasiveness from her based on past encounters and I was surprised at how humble and nice she was over her mistake. So, just ask the teacher how to come up with the answer.
