Once upon a time, there was a poor fisherman in a village near the sea .He always dreamed of having a lo t of money. One day ,he heard that a big ship with a lot of gold sunk in the nearby water.Because of this , for a long time he often went to the sea and hoped to find the gold .

      One day, he was sitting on his boat daydreaming, he suddenly(突然) felt something heavy pulling(拉)on his fishing rod (钓竿).He thought that good luck had come to him , and he pulled hard at it . "Wow! A big, gold chain(金链子)!" He said happily when he saw the thing He then began to pull at the gold chain to try to get it all into the boat, but there seemed to be no end to it. When he got the more gold chain , the more water came into his boat .

       But, the fisherman did not know it .He began to dream of a big house , a big piece of land , buying horses and cows…… He kept pulling in the chain though the boat kept sinking. At last ,the boat went under the sea and the fisherman died (死)

以O为圆心的两个同心圆的半径分别为9cm和5 cm，若⊙P与这两个圆都相切，则下列说法中正确的是(    )．