What question does Winston ask of O'brien? Which question does O'brien refuse to answer and why?

Winston asks about Julia. She has betrayed him, and every part of her independence has been destroyed. Winston asks if Big Brother exists. O'Brien answers that he does, in the way that the Party does, but that he will never die. Winston asks if the Brotherhood exists, and O'Brien says that Winston will never know the answer to that question. Winston asks what is in Room 101. O'Brien answers, "You know what is in Room 101, Winston. Everyone knows what is in Room 101."