All immunity of the state from the provisions of the Sherman Antitrust Act [Act July 2, 1890, c.647; 26 Stat. 209; 15 U.S.C. 1 et seq.] is hereby extended to any city or city governing body acting within the scope of the grants of authority contained in sections 40-05-01, 40-05-02, and 40-05.1-06. When acting within the scope of the grants of authority contained in sections 40-05-01, 40-05-02, and 40-05.1-06, a city or city governing body shall be presumed to be acting in furtherance of state policy.