; (B) delivering a duly executed copy of such demand or petition to the principal office or place of business of the partnership, corporation, association, or entity to be served; or (C) depositing an executed copy of such demand or petition in the United States mails by registered or certified mail, with a return receipt requested, duly addressed to such partnership, corporation, association, or entity at its principal office or place of business. (2) Natural persons Service of any such demand or petition may be made upon any natural person by— (A) delivering a duly executed copy of such demand or petition to the person to be served; or (B) depositing an executed copy of such demand or petition in the United States mails by registered or certified mail, with a return receipt requested, duly addressed to such person at the person's residence or principal office or place of business. (f) Proof of service A verified return by the individual serving any civil investigative demand under subsection (a) or any petition filed under subsection (k) setting forth the manner of such service shall be proof of such service. 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. (g) Documentary material (1) Sworn certificates The production of documentary material in response to a civil investigative demand served pursuant to this section shall be made under a sworn certificate, in such form as the demand designates, by— (A) in the case of a natural person, the person to whom the demand is directed