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

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D
解析 由文章第三段可以得知，1977年人们试验成功，因此试验应该是在1977年之前就已经开始进行了的。
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