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

the answer is c why?

A，it指代不清

B is the best

C，部分人认为C对，但they不能指代proportion，且时态没必要使用完成时，应该是过去时

D， which指代不正确

E ，which指代不正确，it指代不清，时态不好