Only when we saw the photos about the air crash ________ how terrible it was. [ ]

A. had we known  

B. we had known

C. did we know  

D. we knew

目前家庭个人计算机用户大都通过电话线路与计算机网络相连，使用模拟通信。近年来，数字通信已开始在长距离话音和数字、数据领域逐渐替代传统的模拟通信。因此，可以预言数字数据通信最终将取代模拟数据通信。