请教GWD-1-14

GWD-1-14

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 They 没有指代对象，同时不应使用完成时
D. which is greater than was so　which 指代不明
E. which is greater than it has been　which 指代不明

我选了Ａ，　但说不出Ｂ有什么错误．

有说Ａ中ＩＴ没有指代对象．为什么不能指代proportion？

对不起．．．是ＧＷＤ－１－１６．．．

好象发表了就不能改了．．．对不起啊．．．