Scenario 6: You have NO idea what the answer could be.

Even so, you tell the student "That is a very interesting question. However, I do not have an answer for you right now, but I can find out and get back to you during the next discussion session." Then you continue on with the lecture as planned. It is OK not to know the answer to every single question that is asked of you. Remember, even if you are new to a subject, you still know MORE than your students do, so relax.