(a) After district bonds have been voted, the commissioners court shall levy and have assessed and collected on all property in the district taxes sufficient in amount to pay the interest on the bonds and to annually deposit an amount in the sinking fund sufficient to discharge and redeem the bonds at their maturity.
(b) If advisable, the sinking fund shall from time to time be invested by the commissioners court in county, municipal, district, or other bonds which may be approved by the attorney general.