阅读下列材料，从A、B、C、D四个选项中选出最佳答案。

    A young woman traveled and stayed in a hotel. All the rooms in the hotel were singles(单人间). So she lived in a single room.

    Just when she was going to bed, there was a knock at the door. She had no friends in the city. So she was quite surprised but still opened the door.

    The man who was standing at the door looked a little nervous. "Oh! I'm sorry. I thought this was my room," the man said and walked away in a hurry.

    The woman called the police at once. Soon, the man was caught. He was a thief(小偷)! But how did the woman know that?

    All the rooms in the hotel were singles. Who would knock at his own door if he was staying alone? Only a thief would do this. He wanted to see if anyone was inside before he broke into the room.