All bonds shall be payable, under an annual installment plan which shall commence
payments not more than two years from the date of issue. The annual installment plan
may be implemented by the issuance of serial maturity bonds or bonds having mandatory
sinking fund requirements. All bonds shall be made payable within the probable useful
life of the improvement or undertaking with respect to which they are to be issued,
or, if the bonds are to be issued for several improvements or undertakings, then within
the average probable useful life of all such improvements or undertakings. In the
case of a bond issue for several improvements or undertakings having different probable
useful lives, the county council shall determine the average of said lives, taking
into consideration the amount of bonds to be issued on account of each such improvement
or undertaking, and the period so determined shall be the average period of useful
life. The determination of the county council as to the probable useful life of any
such improvement or undertaking shall be conclusive. No bonds shall mature and be
payable more than forty years after their date of issuance except bonds issued under
the authority of The Metropolitan District Act, as amended.
