I don’t know why, but I hate that question (at least in this context). It seems like it is shifting the question of ethics into some sort of meta-ethics. I wonder if you professor was used to formulate an ethical argument, but I personally find it very hard. This might be a good advice in competitive debate, but when you are facing a real argument in the real world where somebody is claiming to be a moral superior I don’t know if this is a helpful advice. E.g. I have no idea how to explain to a nihilist that murdering humans is worse then killing animals for meat. Is that even possible?