What do I do if I don’t know the answer to a question asked during Q & A?

That depends.

If answering the question requires input from a specialist who is not in attendance at the interview:

Let the panel know that the specialist will likely have the best answer to that question and you will contact that individual immediately upon leaving the interview and get back to the contracts administrator via email with an answer as soon as possible.

If  the question is not answerable because the project information currently available is not sufficient to accurately answer the question:

Let the panel know that based upon the information currently available, any response would be speculative.  If appropriate, add the following, “However, we had a similar situation on a previous project and in that instance it made good sense to approach it this way.”

If the question is just confusing:
Ask for clarification.

If you truly have no clue:
See if another member of your team knows the answer.  Never make up an answer because that will likely come back and bite you in the behind.