GMAT Sentence Correction (SC) | Re: Biologists believe that they have found one of the

From the posts, it is not very clear to me, when we can consider "that" referring to a relative plural noun or a phrase. In this case I understand that, "that" is actually referring to substances and hence verb "tell" should be in plural form.

Can you please elaborate this with example which identify use of "that" as relative pronoun modifier and phrase modifier.

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