A、Because he could have time to send war information to the Germans. B、Because it was the best place to wait for his girlfriend

Mike Wilson worked as a low rank official in the War Office during the early 1940s. Though he didn’t bold an important position, he got along very well with almost everybody, and was believed in by most of his leaders.
    One day, Wilson arrived at his office in an expensive car. Little as his pay was, he appeared to have got a lot of money to spend. he bought an expensive house and gave parties one after another. At one of the parties he met a beautiful woman and fell in love with her. When he was asked by the woman one evening how he had suddenly got so much money to spend, Wilson explained that he had a very rich uncle who lived abroad and posted him money nearly every month. But his story could not fool the woman. She was a policewoman and was sent to watch him closely by acting as his "girlfriend", because the police had noticed that he often stayed behind in the evenings and was usually the last person to leave the War Office.
    His "girlfriend" and three other policemen entered his house when he was out and discovered copies of government secret papers and a radio transmitter hidden inside a piano. After Wilson was caught, it was learned that his real name was Jack Brown, and that he had been hired as a spy for the Germans.