Sentence Correction (SC) | Re: Astronomers have discovered what is now the largest known structure -

I eliminated A because it is not following the parallelism "stretching" also present participle is not directly used as the action verb but it modifies the noun or any verb here stretching is one of the main verb but it is used in the present participle. In B I didn't find this error everything is in parallel and also the main verb is not a present participle. so can someone please explain the reasoning which I have used is correct or not?

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