A、He knew this beforehand. B、He judged from his common sense. C、He judged from the speeds of sound D、He judged by reading the te

Professor Wang is a physics teacher in Beijing. Once she was explaining to one of her classes about sound. She decided to test the students to see how well they had understood her. "I have an uncle in Xinjiang." she said. "Suppose I was calling him on the phone from here, and at the same time you were 25 metres away, listening to me. Who would hear me first, my uncle or you? And for what reason?" Ma Hue at once answered, "Your uncle, Professor Wang, because sound- waves travel slower than electricity."
    "Good," said the professor, but then another boy raised his hand.
    "Yes, Li Ming?" said Professor Wang.
    "Of course your uncle would hear you first," Li Ming said, "but that’s because of the time difference. When it’s nine o’clock here, it’s seven o’clock in Xinjiang."