The sentence is
According to a recent survey, the traditional language of the Saanich, a First Nations indigenous community with roots on Canada's Vancouver Island, has fewer than twenty fluent speakers. All of them over the age of sixty.
To replace "speakers. All of them"
B) speakers, all of whom
C)speakers; all of whom
D) speakers, all of them
I know why option A and option C are incorrect. My problem is with options B and D.
The correct answer is option D but I don't understand why option B is ungrammatical or even less favorable. If my understanding is correct, because the second clause has no verb it is a dependent clause. If it did however have a verb only option B would be correct because it would be the only dependent clause because of the relative pronoun whom. Why does omitting the verb make it not work?