询问一个关于 and/but 和comma的知识点

首先是OG diagnostic test上的一个题According to scholars, the earliest writing wasprobably not a direct rendering of speech, but was
more likely to begin as a separate and distinctsymbolic system of communication, and only later
merged with spoken language.
(A) was more likely to begin as
(B) more than likely began as
(C) more than likely beginning from
(D) it was more than likely begun from
(E) it was more likely that it began

对D的解释是 Use of the pronoun it makes this construction a main clause, in which case
the comma after communication must be omitted and began must be used to be parallel to merged; was…begun is not the correct tense.

我觉得你断章取义了......
首先，这是三层意思的平行关系：
1. was not (a rendering of speach)
2. began as (a system of communication)
3. merged with (spoken languages)
你注意到，第三层部分，是非划线部分，那里没有加it，所以第二层也不能加it.

OG的解释是说，假如你像D那样，加了it，那就必须第二层和第三层合在一起，作为整体，和第一层并列，也就是说第二层和第三层之间的逗号应该去掉。像这样：
the earliest writing             was not (a rendering of speach)
, BUT
it                                       began as (a system of communication)
     AND                            merged with (spoken languages)
就说变成了这样一种逻辑关系：  1 BUT (2 AND 3)
姑且不说逻辑上是不是这样.....就算如此，那么后面2和3之间不该有逗号，OG说的就是这么一个意思（GMAT永远没说所有的and前面的逗号都要去掉）。所以D选项是错误的。


你下面那道题，这里有解答http://www.beatthegmat.com/after-several-years-of-rapid-growth-t64579-15.html
其实我认为关键就是2点：
第一，"comma+but"后面没有主语，这不能作为无脑排除E选项的理由，因为逻辑意思很清晰，平行关系很明确，反倒是加上"it"之后不清晰了；
第二，C的表达不对，你可以说"fall behind in its payment to sb." (fall behind in noun)，也可以说"fall behind in paying sb." (fall behind in doing sth.)，但不能说"fall behind in its paying sb.". 这里看上去its paying sb.是个simple gerund，相当于noun，但规则是，如果存在一个action noun来表示这个意思，我们就不要用v-ing (simple gerund)，而要用这个英语中存在的action noun   ——Manhattan后面advanced某章明确讲到。


然后接下来的是GWD上的一道题
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

答案选E无异议，就是对E中的but前面为何依然可以使用comma呢？~

最后一个问题，and　but  or 的用法应该都是一样的吧？~如果这个地方OG上对and有这种强调的话，那么其他两个呢？~