Generally speaking, there is no standard answer to mathematical modeling, and there is a lot of room for modeling. The defense questions mainly focus on your modeling ideas and logical reasoning, that is, how to prove that your model is optimal, and you may ask some questions that you have not considered clearly or explained clearly in modeling. Of course, if your logical reasoning is strong and your assumptions are reasonable, I don't think there is any problem that can baffle you. Wish you success!