GMAT Sentence Correction (SC) | Re: The ancient question of the exact difference between plants and animal

The ancient question of the exact difference between plants and animals, which was so complicated with the discovery of microscopic members of both groups, was somewhat side-stepped with the establishment of a third phylum, the Protista,reserved just for them.
(A) reserved just forthem - Antecedent of THEM is not ambiguous, but use of Plural pronoun "THEM" for singular antecedent "PHYLUM/PROTISTA" is incorrect. (B) consisting only ofthem -As explained in option A
(C) inhabited
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