a Problem 3: Modify the CPL model to take into account that a subset of already existing facilities V S Vi cannot be closed (but can be upgraded). Indicate the current fixed cost and capacity of facility i e Vi as f; and qi, respectively. Moreover, let f." and qi be the fixed cost and capacity if facility i e Vi is upgraded, respectively. The original CPL model is: Minimize , , +, xy, iel jeV2 iel Subject to Ex, = 1, jeV, JEN d, xx,