Some questions are designed to stimulate thought and discussion rather than to elicit a discrete answer. If called on, you should not feel compelled to have one "right" answer!! This idea is very important. Once you realize that it is perfectly acceptable (and sometimes desirable) to not know all of the answers (if you did, what would be the point of going to school?), it can take away a lot of the anxiety associated with contributing or getting called on. While it is okay to not know "the answer", it is nevertheless important for you to attempt to make a contribution to the discussion.
I believe that even if you don't get the question correct, it's important to stimulate your brain! That would make it easier on yourself when you are learning and will give you the desire to learn more! Even if you get the question incorrect, at the end of the day, you are still human and learning which is all that matters!