GMAT Sentence Correction (SC) | Re: Researchers have explored the possibility that dolphins

Dear expert GMATNinja, could you please help explain usage of "as humans" in correct answer (D)?
As far as I'm concern,[as + noun] is right only when subject of the following clause is that noun. For example,
As a teacher, Mary could assign as many exercises as she wants.
In the above example, Mary is a teacher. But in (D), how can dolphins are humans? Please shed some light. Thank you.

Besides, searching on google, I found out that in other GMAT forums, it is said that OA is (E). I
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