一道prep SC题 请教大家~~！

naryi

After several years of rapid growth, the health care company became one of the largest health care providers in the metropolitan area, while it then proved unable to handle the increase in business, falling months behind in its payment to doctors and hospitals.

A. while it then proved unable to handle the increase in business, falling months behind in its payment to

B. while it then proved unable to handle the increase in business and fell months behind in its payment to

C. but then it proved unable to handle the increase in business, falling months behind in its paying

D. but then proving unable to handle the increase in business, falling months behind in paying

E. but then proved unable to handle the increase in business, falling months behind in paying

答案是E，我选的是C 不是说逗号分开的句子，虽然有but连接，但也要有独立主谓吗 E是省略了相同的主语

这两种判断方法怎样区分？跟连词有关，还是跟有无逗号有关，还是看是否动作是同一主语？考虑几个因素

请大家帮助~~谢谢