A steel company has three mills, M1, M2, and M3, which can produce 40, 10, and 20 kilotons of steel each year. Three customers, C1, C2, and C3 have requirements of 12, 18, and 40 kilotons respectively in the same period. The cost, in units of $1000, of transporting a kiloton of steel frame from each mill to each customer is shown in the table.