Almost everyone likes dogs, and almost everyone likes reading stories about dogs. I have a friend. The friend has a large police dog. Its name is Jack. Police dogs are often very clever. Every Sunday afternoon my friend takes Jack for a long walk in the park. Jack likes these long walks very much. One Sunday afternoon a young man came to visit my friend. He stayed a long time. He talked and talked. Soon it was time for my friend to take Jack for his walk. But the visitor still stayed. Jack became very worried. He walked around the room several times and then sat down in front of the visitor and looked at him. But the visitor paid no attention. He continued talking. Finally Jack couldn't stand it. He went out of the room and came back a few minutes later. He sat down again in front of the visitor but this time he held the visitor's hat in his mouth and ran out of the room. Jack is the name of   _  .
a young man a policeman a police dog a good friend