请教一道题

GWD上的

新的13的第一套

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.

1. a greater proportion than it was
2. a greater proportion than
3. a greater proportion than they have been
4. which is greater than was so
5. which is greater than it has been

d e 的 which 指代不清楚，根據題目應該是比率跟比率比較，c的they不能代表比率，a 的it也不能表示比率?? 而且比較級大多用that替代，不知道這樣說對不對??     請指教