Question

I 　　　　the answer to her 　　　　possible, but I was fully occupied then.

A. could explain; if it had been    B. must have explained; were it

C. should have explained; had it been    D. should have explained; should it be

Answer

C

【解析】考查虚拟语气。句意：如果可能的话，我本该给她解释这个答案，但当时我很忙。根据题意可知此处表示与过去事实相反，当条件句中省略if时需用倒装结构，把had提前。主句表示“本应该”，用should have done.故选C。

【名师点睛】

当虚拟条件句的谓语动词含有 were, should, had 时，if 可省略，而将 were, should, had等词置于句首。如：

Should he agree to go there, we would send him there. 要是他答应去的话，我们就派他去。

Were she here, she would agree with us. 如果她在这儿的话，她会同意我们的。

Had he learnt about computers, we would have hired him to work here. 如果她懂一些电脑知识的话，我们会已经聘用他来这里工作了。

本题就是省略if，had提前的情况。