答案：D

题目分析：考查定语从句。kingdom是先行词，表示地点；which作主语或宾语，whose作定语，后面常有名词；that作主语或宾语；where作状语。定语从句three mountains join together and steep mountains reached to the sky不缺少成分，排除A、C；指地点用where，排除B，故选D。