he was at home I would have phoned him then.A. Had I knownB. I had knownC. Would I knowD

问题补充：

he was at home, I would have phoned him then.

A. Had I known

B. I had known

C. Would I know

D. I would know

答案：

(答案→)A. Had I known

B. I had known

C. Would I know

D. I would know

解析：句意：如果我知道他在家，那个时候我就给他打电话了。对过去情况的虚拟，条件句中用过去完成时，主句用would/should/might/could + have + 过去分词。虚拟条件句中有had，should，were时，可以将if去掉，然后把had，should，were提到主语前面，选A。