A person who has been served with a copy of an application or a notice of a proceeding shall have seven days from the date of mailing within which to answer, except as otherwise provided in this section. Where the application was made pursuant to section 2202.8 of this Title, a tenant shall have 15 days from the date of mailing within which to answer. Where a proceeding is commenced by the issuance of an order to show cause by the district rent administrator, a person who has been served with a copy of such order to show cause shall file his answer within the period specified in such order to show cause. Every answer must be affirmed, and an original and one copy shall be filed with the district rent administrator.