A、He knew this beforehand. B、He judged from time zones. C、He made such a judgment so as to Wang. D、He gave such an explanation b

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 under- stood her. "I have an uncle in Xinjiang." she said.  "Sup- pose 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."