The old engineer talked of the difficulty they ___ the tower
问题描述：
The old engineer talked of the difficulty they ___ the tower
A. builtB had building C had builtD build
选B 为什么啊?怎么解释?
答
talked表示谈论已经是过去时了,而建造塔则是在谈论之前,所以用过去完成时,表示过去的过去