GMAT Sentence Correction (SC) | Re: Since the 1930 s aircraft manufacturers have tried to build

An easy B,

So X and So Y is an idiom

wings is repeated to ensure no ambiguity as to what so smoothly and so perfectly shaped is referring to..

Also chekout the review on GMAT Practice tests
http://gmatclub.com/forum/all-gmat-cat-practice-tests-links-prices-reviews-77460-620.html#p1758429

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