Did you see the books that/which I bought her?
That is a question from the grammar test given by Lucy in her YouTube video "How good is your grammar?".
Lucy, the author of the test, says the relative pronoun "that" is the right answer because it introduces the relative defining clause.
My question is: why the relative pronoun "which" is unacceptable in Did you see the books that/which I bought her??
It's not unacceptable - it's fine.
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