The King lived a small house near New York with their child . Sometimes Mr King came back from work very late. When his wife and the child were asleep, he opened the front door of his house with his key and came in very quietly. But one night when he was coming home late, he lost his key. So when he reached his house , he rang the bell. Nothing happened. He rang it again. Again nothing happened. Nobody moved inside the house. Mr King knocked at the bedroom window. He spoke to his wife, he shouted, but she didn't wake up. At last he stopped and thought for a minute. Then he began to speak like a small child. "Mum," he said, "I want to go to the toilet." He spoke quietly, but at once Mrs King woke up. Then he spoke to her, and she opened the door for him. Mrs King woke up because  _  .
she heard the sound of the bell she heard Mr King's voice she heard a knock at the door she heard the voice "mum" like her child's