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    A mother and her young son get into a bus in a city. The bus conductor(售票员) come to them for their money. The mother says, "I want one ticket to Oxford(牛津)."And she gives her money. The conductor looks at the small boy for a few seconds and then says to him, "How old are you, young man?" The mother begins speaking, but the conductor stops her, and the boy says, "I am four years old at home, and two and a half in buses and trains." The mother's face turns red. She has to take money and give it to the conductor.