关于but后主语省略的问题

qsjvicky

223. (30372-!-item-!-188;#058&005388)     (GWD-23-Q23)

Afterseveral years of rapid growth, the health care company became one of thelargest health care providers in the metropolitan area, while it then provedunable to handle the increase in business, falling months behind in its paymentto doctors and hospitals.

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

B. while it thenproved unable to handle the increase in business and fell months behind in itspayment to

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

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

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

求NN解释，为什么E中，but （it）then中的it要省略，看了很多讨论帖，恕小女子愚钝求NN解答！MANHATTAN上不是说", and(but) svo"么
附有一个讨论帖http://forum.chasedream.com/thread-457472-1-1.html 哪位NN懂了求解释！！！感激不尽！！！

洗头发的水

同楼主问，今天这道题也是想不通为啥E中的IT可以省略。BUT和逗号隔开了两个主句，而且后面BUT那句话不是插入语，必然IT不能省略啊。求大神解释！！

木以晨夕

洗头发的水 发表于 2013-7-22 13:11
同楼主问，今天这道题也是想不通为啥E中的IT可以省略。BUT和逗号隔开了两个主句，而且后面BUT那句话不是插 ...

but后面本来是应该加句子的，但是在主语相同的情况下可以省略主语