I would have paid a visit to you,____possible

这里是虚拟语气
原句可以转化为
If _____ possible,I would have paid a visit to you.
这里是与过去事实相反,由主句中would have paid可看出
所以空里应该是 it had been.即原题中的B
如果楼主还是有点不懂可以补一点虚拟语气的知识A不是虚拟语气吗呵呵。。看来楼主虚拟语气不够扎实啊学的if it were是与现在事实相反如果要用if it were,后面的主句必须是would pay但是原句是would have paid这是固定的搭配If ……did，……would do……与现在事实相反If……had done,……would have done与过去事实相反