Question

【题目】(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

Answer 1

【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，此外根据句意可知是被动，用被动语态，故A项是正确的。