单项选择题

Questions 15-17 are based on the following passage. What caused the prices of the telephone system to lower

A．The materials became cheaper.
B．There was more competition.
C．More people used the phones.
D．The phones were connected across the sea.

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单项选择题

单项选择题

Questions 15-17 are based on the following passage. What caused the prices of the telephone system to lower 您的浏览器不支持 audio 元素。