Resignations of elective officers and of the City Manager, Assessor, Attorney, Clerk, and Treasurer shall be made in writing and filed with the Clerk or Mayor, and shall be acted upon by the Council at its next regular meeting following receipt thereof by the Clerk or Mayor, as the case may be. Resignations of appointive officers, other than the City Manager, Assessor, Attorney, Clerk, and Treasurer, shall be made in writing to the City Manager, and shall be immediately acted upon by him.