I have not written my DAT yet (hopefully within the next month), but if I were you, I would take an estimated guess on what you think the answer is and still mark it, that way if you don't have time to go back to the questions you mark, there is still a small percentage you could get the question right as opposed to getting it wrong via not selecting an answer and just marking it. Hopefully we are not put in the situation where marking a question is even a choice.