请教 GWD 1-Q16

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   答案为什么是 C呢？they 对应的是哪部分？为什么用have been 呢？ 请牛牛们指点