According to the passage, people started to make glass ______. According to the passage the new telephone communication system

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问题 According to the passage, people started to make glass ______.
According to the passage the new telephone communication system ______.

选项 A、has been put into actual use
B、was put into actual use in 1977
C、was put on experiment in 1977
D、had been repeatedly experimented on before 1977

答案C

解析 第三段第一句……,the new system was used successfully in an experiment in Chicago in 1977.答案选C。
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