题目

Uncle Li and Uncle Wang are good friends. They live next to each other and their farms are both at the foot of the mountain. So they can help each other. But neither of them likes to use his head. They work hard but they are both poor. Most villagers have built new houses, but they still live in the low and broken houses. They never find out why. Once Uncle Li went to town to buy some medicine for his wife. In the town he heard the apples in a city were expensive. He told Uncle Wang about it as soon as he went back. They decided to carry some apples to the city. They borrowed some mone如图，AB是⊙O的直径，PA切⊙O于点A，PO交⊙O于点C，连结AC、BC. 若∠BAC=2∠BCO，AC=3，则PA的长为 ( )A. 3 B. 4 C. 5 D. 6