error involved- parallelism.
what are the elements that are being compared? the non underlined part of the sentence gives us a clue regarding this. the elements in the non underlined part of the sentence are nouns. Thus the underlined sentence must contain nouns to keep the sentence parallel.
a substantial decrease and an increase are both nouns and thus are parallel to the rest of the sentence.
IMO D. parallelism issue
In Gmat always avoid unnecessary +ing (gerunds) except some cases, so A,B, and C are out.
if u see the sentence a+noun being used after commas, so only D & E are close choices but in E "substantial" is omitted which doesn't complement the original sentence's meaning. so D is the best choice
OA ? source is www.800score.com's free CAT?