The correct answer is (D).
Getting to the Answer:Examine the entire clause in which the underlined portion appears. The Erie Canal did two things: "connected New York City with ..." and "turned villages ...." The two phrases of this compound must be in parallel form and convey the intention of the sentence: the villages became boomtowns.
(A) Incorrect: "turned villages ... that were ..." is not in the same grammatical form as "connected New York City with ..." and confuses the intended meaning by stating that Rochester and Buffalo were already boomtowns
(B) Incorrect: "turned villages ... which became ..." is not in the same grammatical form as "connected New York City with ..."
(C) Incorrect: although "turned villages ... around ... " is in the same grammatical form as "connected New York City with ... ," this choice misinterprets the meaning of the sentence; the intention of the sentence is that Rochester and Buffalo became boomtowns, not that they were "around," or near, boomtowns
(D) Correct: "turned villages ... into ..." is in the same grammatical form as "connected New York City with ..." and conveys the intended meaning of the sentence
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