We don't use 'that' in non-defining relative clauses, so we need to use 'which' if the pronoun refers to some matter, and 'who' if it refers to your person. We can't fall the relative pronoun in this sort of clause, regardless of whether the relative pronoun is the thing of http://sam-amoo18495.educationalimpactblog.com/34118001/the-best-side-of-contactor