答案：A
翻译：如果可能的话,我本来早就告诉他答案了,但是我当时那么忙.
1.从句意来看,逗号前的句子是对过去发生的动作进行的虚拟,即：
I would have told him the answer had it been possible
= I would have told him the answer (if it had been possible)
2.而逗号后面的句子当然也就是对过去发生的事实进行的描述,再根据其时间状语then,所以只需用一般过去时即可,即答案A为正解.
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