"What did the horse do during the final week?"
Translation:말은 마지막 주 동안 무엇을 했어요?
it's not that. in the same section they accept both 작년에 뭐 했어요 and 작년에 무엇을 했어요. so that they refusee 뭐 and 뭘 here makes no sense.... but I have my answer to be honest: Duolingo is full of mistakes, inaccuracies and inconsistency. they should not have put the Korean lessons online if they were not ready.