A、By asking some students to explain the reason. B、By giving them a test on the speed of sound. C、By using a vivid question to c

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."