It is not necessary.
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AnonymousWhy not?The more common question when referring to grammar is Why? To which, of course, the answer is simply Because. In this case, I can tell you that sometimes the relative pronoun (and ensuing verb) are simply elided. The dog
AnonymousOn the table there are five magazines. In which only three mention Kate.Much more natural: On the table there are five magazines, only three of which mention Kate. [ in only three of which is Kate mentioned - but this is awkward, as passive voice tends to be ]