While Nick was on the bus, reading his newspaper, the man sitting next to him suddenly pushed a large envelope into his hands. "Here, take this!" the man said, stood up and got off the bus before Nick could say a word. Nick sat there, holding the envelope. It felt heavy. There were papers inside, or money perhaps. "I'd better hand it over to the police," he thought. There was a police station close to his office. But, as he got off the bus, a man came to him. He seemed to be waiting for something. "He wants the envelope," Nick thought. Nick began to walk quickly, and the man hurried after him. Nick started to run, and the man began to run, too. But then, just before he got to the police station, Nick managed to lose the man in the crowds. When he entered the police station, the man was no longer in sight. Inside the police station, Nick handed over the envelope to a policeman in charge. The man opened it. The envelope was full of money, false money. "Clearly the man made a mistake," the policeman said. "He thought you were one of the gang  !" Nick felt like a hero. He could already see his name in all the papers. He could imagine an interview on television. "However," the policeman went on, "I'm afraid I must ask you to keep quiet about all this. We're trying to catch some very clever thieves, and we don't want them to know that we have some of the money. So you mustn't say a word to anyone -- not even your boss!" As Nick got off the bus, a man came to him because   _  .
he wanted to catch Nick he thought Nick was a policeman he wanted to give Nick some money he thought Nick was one of their gang