单选题 How long ______ each other when they got married?
A. had they known B. have they known
C. would they know D. did they know

【正确答案】 A
【答案解析】结婚的事情已经发生，是过去时，到他们结婚时认识了多久应用完成时，而他们的结识是在结婚前。所以，应用过去完成时。选项A是正确的。