关于but用法的一个疑问

stellalights

曼哈顿中说 主语发出的两个动作and前不需要加逗号，如i went to the supermarket and bought mant stuff.如果重复主语就要加逗号i went to the supermarket, and i bought mant stuff.
那么but是不是一样的用法呢。
今天做到prep上的一道题时，

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. whileit then proved unable to handle the increase in business, falling months behindin its payment to

B. whileit then proved unable to handle the increase in business and fell months behindin its payment to

C. butthen it proved unable to handle the increase in business, falling months behindin its paying

D. butthen proving unable to handle the increase in business, falling months behindin paying

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

选E，这里but后面没有重复主语，却加了逗号，语法笔记上说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。
难道和and的用法不一致吗？？

duoleshi

这也是我的疑问，而且这题还存在proved这个词是主动表被动意思，相当绕人

bejamin1111

观察很仔细哦  

stellalights

walt77 发表于 2013-3-29 17:53
After several years of rapid growth, the health care company became one of the largest health care p ...

明白了 ~还是有特例的~

walt77

After several years of rapid growth, the health care company became one of the largest health care providers in the metropolitan area, but then proved unable to handle the increase in business, falling months behind in paying doctors and hospitals.
粗体字部分，过于笼长，去掉逗号的话，让人难以阅读。

the comma is sometimes employed when “but” is used to connect units smaller than clauses, if those units are so long that the sentence becomes difficult to read without the comma.

for instance:
James tried to sneak back into the house at 5 a.m. without waking his parents, but climbed through a window directly into the room where his father was already getting dressed.
--> try taking out the comma; if you do, the sentence becomes an unreadable blob.

请楼主参考http://www.manhattangmat.com/forums/after-several-years-of-rapid-growth-t5788.html
这个页面，倒数第三楼 Ron 的解释