(含解析)
1.【2017·北京卷】If the new safety system _______ to use, the accident would never have happened.
A. had been put
B. were put
C. should be put
D. would be put
【答案】A
【解析】句意：如果这个新的安全系统被投入使用过的话，这个事故就不会发生了。根据主句确定是对过去情况的虚拟，所以从句谓语用had done形式，此处根据句意可知是被动，用被动语态，故选A。