题目

One morning more than thirty years ago, I entered the Track Kitchen,a restaurant where everyone from the humblest(卑微的) to the most powerful came for breakfast. I noticed an empty chair next to an elderly, unshaven man, who looked somewhat disheveled. He was wearing a worn-out hat and was alone. I asked if I might join him. He agreed quietly and I sat down to have my breakfast.     We cautiously began a conversation and spoke about a wide rang of things. We never introduced ourselves. I was concerned that he might have no money and not be able to afford something to eat. So as I 如图，已知在半圆AOB中，AD=DC，∠CAB=30°，AC=2，求AD的长度．