快考了新prep求助197

197.
            (27650-!-item-!-188;#058&002540)     (GWD-1-Q16) A B

Soaring television costs accounted for more than half the spending in the presidential campaign of 1992, a greater proportion than it was in any previous election.

A. a greater proportion than it was

B. a greater proportion than

C. a greater proportion than they have been

D. which is greater than was so

E. which is greater than it has been

已查gwd的解释贴，这里有个疑问。

gwd上说it不能指代前面整句话，我想问的是：it为什么不能指代proportion呢？这样比较对象不是一致了吗？

求教，谢谢！

一般来讲，比较时间上连续的东西才能用it代替，比如economy.

而proportion明显不是时间上连续的东西，此proportion和彼proportion是完全分得开的，所以不能用it。