【题目】If the new safety system ________ to use, the accident would never have happened.
A. would be putB. were put
C. should be putD. had been put
在线课程【答案】D
【解析】
考查虚拟语气。句意：如果新的安全系统投入了使用，那么这场事故永远也不会发生了。根据语境以及主句the accident would never have happened的形式可知，该题是对过去的虚拟。表示对过去的虚拟，条件句的谓语部分应用过去完成时态。故D选项正确。