Bob would have helped us yesterday.but he ____.

问题描述:

Bob would have helped us yesterday.but he ____.
A.was busy B.is busy C.had been busy D.will be
为什么选A呀?这个动作发生在过去,应该用过去完成时呀?不是吗

would have done是对过去时的虚拟,后面句子用过去时