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A．In Boston.
B．In America.
C．In the Peak District.
D．In a big world.
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单项选择题
A．Use of copper conductors.B．Use of plastic insulator (绝缘体).C．Use of silicon (硅) semiconductors.D．Elimination of chemicals.
A．Use of copper conductors.
B．Use of plastic insulator (绝缘体).
C．Use of silicon (硅) semiconductors.
D．Elimination of chemicals.
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