Notices may be served upon agents, owners and occupants in any of the following ways:
(A) By personal delivery;
(B) By mailing to such person's last known address;
(C) In case of a vacant business building or part thereof, when the agent or owner of same cannot be found within the county after due diligence, or his mailing address cannot be determined, then a notice posted upon the premises in some conspicuous place will be deemed service upon such agent and owner.
('70 Code, § 11-22)