[求助]TT GWD-4-19?????

Q19:

After several years of rapid growth, the healthy 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.

1. while
        it then proved unable to handle the increase in business, falling months
        behind in its payment to
2. while
        it then proved unable to handle the increase in business and fell months
        behind in its payment to
3. but
        then it proved unable to handle the increase in business, falling months
        behind in its paying
4. but
        then proving unable to handle the increase in business, falling months
        behind in paying
5. but
        then proved unable to handle the increase in business, falling months
        behind in paying

  
Answer:E

I choose A, why E? E中but分句没有主语