A national catalog and Internet retailer has three warehouses and three major distribution centers located around the country. Normally, items are shipped directly from the warehouses to the distribution centers; however, each of the distribution centers can also be used as an intermediate transshipment point. The transportation costs (S/unit) between warehouses and distribution centers, the supply at the warehouses (100 units), and the demand at the distribution centers (100 units) for a specific week are shown in the following table: Distribution Center Warehouse A B C Supply $12 $ 11 $ 7 70 6 14 80 W N D 00 10 12 50 Demand 60 100 40 The transportation costs ($/unit) between the distribution centers are Distribution Center Distribution Center A B C $- $8 $3 N Required: Please prepare the objective and constraint models for this