请问这道题考察的什么知识点?请给我讲讲吧
问题描述：
请问这道题考察的什么知识点?请给我讲讲吧
＿＿for your help,I'd never have been able to get over the difficulties.
A.Were it not B.Had it not been C.If it were not D.If it had been
答
B
If 引导的虚拟语气,If在前句首倒装
与过去事实相反,从句用过去完成时