题目

     Once, when Tom was a little boy, his mother went out for a picnic. Before she left, she said to him. Tom, while I am away, stay near the door and watch it all the time! She said this because there were a lot of thieves in their town.      Tom sat down near the door and watched it. After an hour, one of his uncles came, and he asked Tom, Where's your mother? Tom answered, At a picnic. Well, said his uncle. We' re going to visit your house this evening. Go to tell your mother.      His uncle then went away, and Tom began to think, Mother said, ' Watch the door well all the time如图，点D在△ABC的边AC上，将△ABC沿BD翻折后，点A恰好与点C重合，若BC=5，CD=3，则BD的长为（　　） A、1B、2C、3D、4