They arrived at the farmhouse, in front of which .
A. is sitting a boy B. sat a boy
C. a boy sat D. a boy is sitting
They arrived at the farmhouse, in front of which .
A. is sitting a boy B. sat a boy
C. a boy sat D. a boy is sitting
选B。地点状语放在句首构在完全倒装，即主语与谓语直接颠倒位置，不加助动词。选A、D与主句时态不符，选项C应构成倒装句才对。