Upon completion of the precise plans and specifications, the work or any portion of it may be done in any of the following ways as ordered by the district board:
(a) By purchasing the material and doing the work by day labor.
(b) By purchasing the material and letting a contract for the doing of the work.
(c) By purchasing only a portion or none of the material and letting a contract for furnishing the balance or all of the material and the doing of the work.