This question tests a common comparison idiom: "not by...but by...." When the first is present, the second must be, as well. That eliminates (A) and (B), both of which use "but" instead.

Purely for style reasons, (C) is preferable to (D) and (E). The latter two choices restate the words "brought about," which is superfluous when compared to the construction used in (C). In addition, (D) is wrong because "as was" (in (C)) is more accurate than "which was." "As was the case" is referring to an action, while "which was" refers to a thing. The substantial influx was an occurrence, not a thing. (E) is wrong because it compares "immigrants" (as opposed to "a substantial influx") to "adjustments to the federal tax code." In (C) and (D), the comparison is between social phenomena.