Why not? What do you think is a good answer?
Because it doesn't show confidence in your answer. You can't say you are confident in your decisions by answering, "I think so."
This happened to me...luckily, I reacted quickly, made a joke and we all laughed. Then again, interviewing is one of my best skills. My best suggestion for the interview is to not be nervous and have some fun with it.
A good part of my interviewing was spent laughing with the controller and the HR lady.