Bonds issued under this chapter may be secured by:
(1) solely a pledge of revenues, in the manner and to the extent prescribed for a county by Chapter 284, Transportation Code;
(2) a pledge of, and making the bonds payable from, an ad valorem tax authorized by Section 52(b), Article III, Texas Constitution;
(3) designating a portion of the bonds to be secured solely by a pledge of revenues as provided by Subdivision (1) and a portion of the bonds to be secured by a pledge of an ad valorem tax as provided by Subdivision (2); or
(4) a combination of the methods prescribed by Subdivisions (1) and (2) through which all of the bonds are to be supported and secured by a pledge of revenue and ad valorem tax, in the manner and to the extent prescribed for a county by Chapter 284, Transportation Code.