My question has to do with reduced relative clauses. I have a textbook (Making Connections by Pakenham) for ESL learners in academic settings in the US which has this example of a relative clause (underlined): A sound track that contained the same conversation with the same two voices was added to each videotape. Now, the text states that this example can be reduced to the following: A sound track containing the same conversation with the same two voices was added to each videotape. My question is: why does it change from the past participle contained to the gerund (?) containing when the clause is reduced? Thanks!