What are some of the starting phrases that I can use while responding to Ph.D examination committee questions during my defence?
In addition to Pete Clark's excellent answer, I would like to offer a second piece of advice.  When you are feeling nervous or "put on the spot" by questions, an excellent first step is to begin by ensuring that you have understood the question.  You can say something like:


  "If I understand correctly, you are asking [paraphrased question]


and then go on with your response.  This both makes sure you are clear about your communication as well as giving you a moment to steady your nerves and to think about your response.