请教GWD3-Q27

GWD3-Q27:

Competition in the mid-nineteenth century by large western farms gradually caused famers 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 famers 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 famers to trun to livestock raising.

C. Pennsylvania's famers were gradually caused to turn to livestock raising by competition from large western farms in the mid-nineteenth century, but before that is was predoninantly 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. Pennylvania 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, 为什么A不对呢?请NN指点一下.谢谢!!

在答案A中，it没有指代对象，如果it指代Competition明显不符合逻辑。