(c) The funds in the general obligation debt sinking fund and the guaranteed revenue
debt common reserve fund shall be as fully invested as is practicable, consistent with the
requirements to make current principal and interest payments. Any such investments shall
be restricted to obligations constituting direct and general obligations of the United States
government or obligations unconditionally guaranteed as to the payment of principal and
interest by the United States government, maturing no longer than 12 months from date of
purchase.