Except as provided in subsection (c) of this section, in accordance with the authority to issue variable interest rate bonds under § 8-121 of this subtitle, the Board, in a resolution setting forth the terms and conditions of a State bond issue, may designate that all or a portion of the State bonds be issued as variable interest rate bonds. The Board may sell variable interest rate bonds at a private, negotiated sale on the terms and conditions that the Board determines to be most advantageous to the State. At the time of issuance, the aggregate par value of the State’s general obligation variable interest rate bonds shall comprise no more than 15% of the outstanding general obligation indebtedness of the State.