; or (C) depositing a duly executed copy in the United States mails, by registered or certified mail, return receipt requested, duly addressed to such person at the principal office or place of business of such person. (9) Proof of service (A) In general A verified return by the individual serving any civil investigative demand or any enforcement petition filed under this section setting forth the manner of such service shall be proof of such service. (B) Return receipts In the case of service by registered or certified mail, such return shall be accompanied by the return post office receipt of delivery of such demand or enforcement petition. (10) Production of documentary material The production of documentary material in response to a civil investigative demand shall be made under a sworn certificate, in such form as the demand designates, by the person, if a natural person, to whom the demand is directed or, if not a natural person, by any person having knowledge of the facts and circumstances relating to such production, to the effect that all of the documentary material required by the demand and in the possession, custody, or control of the person to whom the demand is directed has been produced and made available to the custodian. (11) Submission of tangible things The submission of tangible things in response to a civil investigative demand shall be made under a sworn certificate, in such form as the demand designates, by the person to whom the demand is directed or, if not a natural person, by any person having knowledge of the facts and circumstances relating to such production, to the effect that all of the tangible things required by the demand and in the possession, custody, or control of the person to whom the demand is directed have been submitted to the custodian. (12) Separate answers Each reporting requirement or question in a civil investigative demand shall be answered separately and fully in writing under oath, unless it is objected to, in which event the reasons for the objection shall be