第29题: [单项选择]According to the passage the new telephone communication system ______. 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 参考答案:D 答案解析:[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。