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 greater proportion than it was

a greater proportion than

a greater proportion than they have been

which is greater than was so

which is greater than it has been

参考答案C

可是我觉得应该选B，因为无论是it还是they都是找不到指代对象的，大家怎么看？

They指代Soaring television costs

时态要补出吧。