Miss. Green taught Physics  in a London school. Last month she explained to one of her classes about sound, and she decided to test them to see how well she did in her work. She said to them, "Now, I have a sister in Washington. If I was calling her on the telephone, and at the same time you were 20 metres away, and listened to me from the other side of the street, who would hear what I said earlier, my sister or you? And why?" The cleverest boy answered at once, "Your sister, Miss. Green, because electricity   travels faster than sound waves." "That's very good." Miss. Green said. But then one of the girls put up her hand. Miss. Green said, "Yes, Betty?""I don't think so," Betty said. "Your sister would hear you earlier because when it's eleven o'clock here, it's six o'clock in Washington." Miss. Green was teaching the class  _  .
about time about sound about electricity about telephone