GMAT Sentence Correction (SC) | Re: Unlike the virginal, whose single set of strings runs parallel to the

I understand that D is correct since we are comparing virginal to the harpsichord..

but for the SV agreement for "single set of strings that run parallel" is it incorrect to reduce it down to "single set that runs parallel"?
i'm gonna guess that it's the "strings that run parallel" and not the "single set runs parallel"

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