(a) The bonds shall be executed by the manual or facsimile signature of the chairman of the board and by the manual signature of the secretary of the board.
(b) The coupons attached to the bonds shall be executed by the facsimile signature of the chairman of the board.
(c) In case any of the officers whose signatures appear on the bonds or coupons shall cease to be such officers before the delivery of the bonds or coupons, their signatures shall, nevertheless, be valid and sufficient for all purposes.
(d) The district shall adopt and use a seal in the execution and issuance of the bonds, and each bond shall be sealed with the seal of the district.