The State Treasurer shall invest all sinking fund monies in direct obligations of the United States of America or in certificates of deposits from banks in the State of Oklahoma acceptable as depositories by the State Treasurer, when such
certificates of deposits are secured by acceptable collateral and yield as much or more than direct obligations of the United States of America, to mature in time to meet the principal and interest payments on the Series D Bonds, which earnings shall be deposited in the sinking fund. Laws 1968, c. 265, § 10, emerg. eff. April 29, 1968.