Which or that?

Whether it’s “which” or “that” depends on whether you want to listen to the convention police. You’re not wrong if you ignore it, but the convention is to reserve “which” for non-defining relative clauses (“The couch, which has a stain on it, is dirty”), and “that” for defining relative clauses (“Here is a couch that has a stain on it”). If it’s between commas or after a comma, err on the side of “which”.