The Nursery manufactures bags of potting soil from compost and topsoil. Each cubic foot of compost costs 12 cents and contains 4 pounds of sand, 3 pounds of clay, and 5 pounds of humus. Each cubic foot of topsoil costs 15 cents and contains 4 pounds of sand, 6 pounds of clay, and 11 pounds of humus. Each bag of potting soil must contain at least 13 pounds of sand, at least 12 pounds of clay, and at least 10 pounds of humus. (Hint: This is an LP problem and here objective is to minimize the cost. There are three constraints, amounts of sand, clay and humus.) a. What are the decision variables? b. Write the complete objective function. c. Write the constraints