Here is the explanation
The best way to solve this sentence is through removing some of the additional information that it provides as a means to get to its core. Therefore, take out -nearly 21% of the GDP in 2000- when reading the possible responses. This will make it clear that we need to say, “federal revenues equaled more than they equaled” or “federal revenues equaled more than they did.”
(A) Correct.
(B) Singular pronoun “it” does not agree with the plural referent “revenues”
(C) “They were” implies “were equaling,” but since “equaling” is nowhere in the sentence it cannot be implied.
(D) “Higher” cannot be used to describe “equaled,” instead the adverb more should be used.
Parallelism: in order to maintain parallel structure, both sides of the comparison need to be written in the same way (see above). The correct answer needs to say “than they did”
(E) “Higher” cannot be used to describe “equaled,” instead the adverb more should be used.
“They were” implies “were equaling,” but since “equaling” is nowhere in the sentence it cannot be implied.
The correct answer is A
i hope this helps in clarifying any doubt