Numerous studies have shown that the income levels of working adults

OFFICIAL EXPLANATION:

The original sentence correctly makes a comparison between the income levels of working adults who were average students and the income levels of students who were exceptional students. These two elements are logically parallel, and thus should be structurally parallel. However, this sentence is problematic in its use of the term "those adults," since the pronoun "those" is both unnecessary and not parallel in this context. Also, the phrase “of exceptional academic abilities” is not precisely parallel to the phrase “of average academic ability” in the non-underlined portion of the sentence. As this sentence makes a comparison, the two elements should be as parallel as possible.

(A) This choice is incorrect as it repeats the original sentence.

(B) This choice correctly uses "those" to refer to income levels, enabling a correctly framed comparison. However, it incorrectly uses the past perfect tense "had been," which is unjustified by the sentence and is not parallel to the simple past tense "were" used earlier to describe students of average academic ability. Finally, “exceptionally able students academically” is both unparallel and awkward.

(C) CORRECT. The pronoun "those" is correctly used to refer to income levels, enabling a properly drawn comparison. Additionally, the simple past tense verb "were" is parallel to the verb "were" in the non-underlined portion of the sentence.

(D) This choice incorrectly compares income level to adults, rather than the proper comparison between income levels and income levels.

(E) This choice incorrectly uses the past perfect tense “had been," which is unjustified and also not parallel to the non-underlined simple past tense verb "were."