Today my English teacher(not a native) used the sentence below to introduce the idea of relative clause:
People will buy the classics based on her recommendation but sales won't reach the kind of numbers achieved in the first book club.
My teacher explained that the sentence above is a relative clause because which were is originally placed before achieved , and numbers is modified by "which were achieved in the first book club."
People will People will buy the classics based on her recommendation but sales won't reach the kind of numbers (which were) achieved in the first book club.
My teacher said that which were is omitted in the sentence because grammar rule allows, and the omitted which represent numbers. So, the relative clause part of the sentence can be seen as : Numbers were achieved in the first book club.
But my friend told me that the teacher is partly wrong because there is no such thing as omitting which were or that "numbers which were achieved" is transformed into "numbers achieved",
the term for this is called participle phrase. But my friend said that relative clause performs the same function as participle phrase, and the meaning does not differ, so the teacher isn't completely wrong.
So I am really confused because I was given two different thoughts,
at this point I don't know whom to trust, which one is right and which is wrong?