Bunuel
The consultant team advised the insurance company that, in order to remain profitable through the upcoming decade, it had to diversify its long-term equity holdings, streamline its now labyrinthine claims processing systems,
and to balance the added liability of senior citizens' living longer by changing its premium structure.
A. and to balance the added liability of senior citizens' living longer by changing
B. and balance the addition of liability with senior citizens living longer by changing
C. and balance the added liability of senior citizens living longer with changes to
D. balancing the added liability with senior citizens living longer to change
E. thereby balancing the addition of liability with senior citizens living longer with changes to
Magoosh Official Explanation
Split #1: parallelism! We have three verbs, in the infinitive form, in parallel. We have two options: we could have the word "to" repeated in all three cases, or we could say the word "to" once at the beginning, and have it apply equally to all three terms:
option #1: "to diversify …, to streamline …, and to balance …"
option #2: "to diversify …, streamline …, and balance …"
Notice, the word "streamline" in the non-underlined portion does not have the word "to" in front of it: therefore, option #1 is impossible in this sentence, so we have to choose option #2. Having the "to" in front "balance", as in (A), is incorrect. Choices (D) & (E) have the participle "balancing", which completely violates the parallelism, so these are incorrect as well. Only (B) & (C) complete the parallelism correctly.
Split #2: the correct idiom is "balance X with Y." Here, we want to balance "liability" with "changes", so the preposition directly in front of the word "changes" should be the word "with." Only (C) & (E) have this correct.
Split #3: the phrasing "added liability" is certainly more concise than "the addition of liability", and arguable the latter changes the meaning slightly as well. For both reasons, we have to reject the latter, which appears in (B) & (E): those are incorrect.
The only answer that is both grammatically and idiomatically correct is (C).