请问这道虚拟语气的英语选择题怎么做?

Had I known about this computer program,a huge amount of time and energy_____.
A.would have been saved B.had been saved C.will be saved D.was saved
请说一下理由?
答：A
译：要是我当时了解这个计算机程序的话,就会省掉大量的时间和精力了.
析：虚拟语气,虚拟过去,倒装省略if的用法.
我们知道,与过去事实相反的假设,虚拟条件句用if 主+had done sth,主句用would +have done sth.,从句中如有助动词had,(should,were),可能通过提前而省略if,所以,Had I known ...= If I had known ...,.