GMAT Sentence Correction (SC) | Re: By the same techniques used for genetically enhancing

Hello y'all,

I've a question, why isn't C wrong
C. Employing the same techniques used to genetically enhance plants so that they are

Ignoring the ambiguity of the pronoun "they", one big question arises that what is "used" a verb doing by it self in the phrase which later modifies researchers. any help would be appreciated.

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