Much time___ sitting at a desk, office workers are generally troubled by health problems. A. being spent B. having spent C.spent D.spending 这道题目是独立主格结构，选择C，但是我疑惑的是为什么不是workes为spend动作的逻辑主语，我可以认为是workes spend time，花费时间的动作是工人发出的，也符合逻辑啊，但是为什么一定要按照独立主格理解呢？