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 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