For this problem, what should we do in the model if we want to prevent transporting gasoline from refinery i to distribution area j?

Problem 3. Three refineries with daily capacities of 6, 5, and 8 million gallons, respectively, supply three distribution areas with daily demands of 4, 8, and 7 million gallons, respectively. Gasoline is transported to the three distribution areas through a network of pipelines. The transportation cost given in the following table is provided in the unit of thousand dollars per million gallons. Distribution area 1 2 3 18 35 30 10 8 20 25 12 Refinery 1 Refinery2 Refinery 3 12