题干
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(点此获取答案解析）
A
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
