Sentence Correction

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) a greater proportion than it was
B) a greater proportion than
C) a greater proportion than they have been
D) which is greater than was so
E) which is greater than it has been

OA is B
I got this correct but would love your explanation.. as GMATPrep doesn't provide one . Ultimately I think most ppl would waver between choices A and B.

Soure: GMATPrep

We can eliminate D and E for using the relative pronoun "which" because it's not 1992 that is "greater than..."

C can be eliminated for using the present perfect "have been" to describe past elections.

A uses "it was." What was a greater proportion than in any previous election? Soaring TV costs. Costs are plural and "it was" is singular, so we have an agreement error.

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) a greater proportion than it was
B) a greater proportion than
C) a greater proportion than they have been
D) which is greater than was so
E) which is greater than it has been

IMO: B