(a) Filing and service. Within 20 days after service of the complaint, or such other time as may be specified therein, the respondent shall file with the Hearing Clerk, an answer, signed by the respondent or the respondent's attorney. The answer shall be served upon the complainant by the Hearing Clerk.

(b) Contents. The answer shall clearly admit, deny, or offer an explanation in response to each of the allegations of the complaint, and shall clearly set forth any affirmative defense.

(c) Default. Failure to file an answer shall constitute an admission of the allegations in the complaint, and may be the basis for a decision upon the presentation of a prima facie case by the complainant.