A simple transportation network is shown in the figure below. Factories 1, 2 and 3 are producing certain products to supply for warehouses 1 and 2. The arrows represent the transporting path between factories and warehouses and the numbers on the arrows represent the cost of shipping each unit product on that path. For example, from factory 1 to warehouse 2, the cost of shipping is $6/unit. The supply capacities of factory 1, 2 and 3 are 50, 40 and 60 units, and demand requirements of warehouses 1 and 2 are 70 and 80 units respectively, as shown in the figure. For example, factory 1 can produce 50 units and warehouse 1 needs at least 70 units. Please construct a LP model to minimize the total shipping cost based on the information above. (Assume thatA, andBindicate the quantities shipped from Factory 1 to Warehouse 1 and 2 respectively;CandDindicate the quantities shipped from Factory 2 to Warehouse 1 and 2 respectively;EandFindicate the quantities shipped from Factory 3 to Warehouse 1 and 2 respectively)

What is the supply constraint for Factory 1?