Only after__ for three hours in the train hours in the rain did the reach the destination.
问题描述：
Only after__ for three hours in the train hours in the rain did the reach the destination.
A.they had walked B.had they walked C.they walk D.do they walk
答
选A.
Only after they had walked for three hours in the train hours in the rain did they reach the destination.
后面部分是倒装,前面部分不用倒装；后面部分是一般过去时,且有for three hours这段时间的延续,说明前面部分应该是过去完成时.因此只有A选项符合语法以及语义.