We do not use 'that' in non-defining relative clauses, so we need to use 'which' If your pronoun refers to your matter, and 'who' if it refers into a person. We won't drop the relative pronoun in this kind of clause, even though the relative pronoun is the item on https://samamoo83727.activoblog.com/13165229/not-known-factual-statements-about-contact-irs