英语虚拟语气选择题应该怎么选择时态

英语虚拟语气选择题应该怎么选择时态
i can't stand him.he always talks as though he __everything
A knew B knows C has known D had known
总是区分不开什么时候应该用过去时什么时候应该用过去完成是,求详解,.
i would have told him the answer had it been possible but i __so busy then。
A had been
B were
C was
Dwould be