Suppose that a computer supplier has two warehouses, one located in city A and another in city B. The supplier receives orders from two customers, one in city C and another in city D. The customer in city C orders 50 units, and the customer in city D orders 60 units. The number of units at the warehouse in city A is 70, and the number of units at the warehouse in city B is 80. The cost of shipping each unit from A to C is 1, from A to D is 2, from B to C is 3, and from B to D is 4.

(a) Formulate the problem of deciding how many units from each warehouse should be shipped to each customer to minimize the total shipping cost. Show the cost or objective function and all the constraints.

(b) Express the problem as an equivalent standard-form linear programming problem.