The respondent may file by mail, by personal delivery, or via the commission’s electronic filing system and serve upon the complainant, within 20 days after service of the order, an answer alleging that it has already granted the relief demanded or setting up any matter of defense. If the answer alleges the granting of the relief, the complainant shall within 20 days reply admitting or denying the allegation. If the complainant fails to reply or admits the allegation, the proceeding must be dismissed.