Sentence Correction (SC) | Re: Sentence Correction

I got the right answer and went with E, but i want to make sure my reasoning is correct. Since "whom" is a relative pronoun i would much rather like to use it next to noun its referencing rather than as object for prepositional phrase, on the other hand "them" can be used with a problem.
Any comments on this.

B.migrate, swimming from Southern California to the North Pacific; some ofwhom
E.migrate, swimming from Southern California to the North Pacific, and some ofthem
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