请教一道题 GWD-3-Q27 多谢！

CloverJackier

Competition in the mid-nineteenth century by large western farms gradually caused farmers in Pennsylvania to turn to livestock raising, but before that it was predominantly grain-producing.

A.Competition in the mid-nineteenth century by large western farms gradually caused farmers in Pennsylvania to turn to livestock raising, but before that it was  predominantly grain-producing.

B.Once predominantly a grain-producing state , competition in the mid-nineteenth century from large western farms gradually was causing Pennsylvania's farmers to turn to livestock raising.

C.Pennsylvania's farmers were gradually caused  to turn to livestock raising by competition from large western farms in the mid-nineteenth century, but before that it was predominantly a grain-producing state.

D.It was  once predominantly grain-producing, but competition in the mid-nineteenth century by large western farms was gradually causing Pennsylvania's farmers to turn to livestock raising.

E.Pennsylvania was once a predominantly grain-producing state, but competition in the mid-nineteenth century from large western farms gradually caused the state's farmers to turn to livestock raising.

答案是E,请教一下为什么A不对，有的分析说that it指代不清，但是我觉得that it指代了pennsylvania不是很合适吗？
而且A的主要意思是说competition引起改变，如果是选E不是就改变了整个句义变成了pennsylvania从以前怎么的变成了现在怎么的了吧？
多谢各位解答

Vickyliu

恩。我想先抛开语法来说一些自己的理解～
A选项大概说，竞争让农民转去养牲畜了，但是在那以前P这个地方主要是种谷物的。
E选项大概说，P这个地方以前是种谷物的，但是竞争让农民转去养牲畜了。
两相比较，显然E选项所表达的两个句子间的转折逻辑关系更为明了。
另外，OG12里有个题目强调过关于写作句子时要注意时间顺序的问题。也就是说按照先发生什么，再发生什么这样来把句子写清楚。所以比较A和E，觉得E更为符合常识和逻辑。
以上是一点拙见～还请指正

sbjqbS

那It指什么?

CloverJackier

多谢，请问有高人能从语法方面解释一下吗？