For a perfectly competitive industry or firm, which of the following is always correct? O) A. In the short run, the supply curve is the entire marginal-cost curve. O B. In the long run, a constant cost industry has a downward sloping supply curve. () C. In the short run, the supply curve is the marginal-cost curve below the average variable cost. O D. In the long run, a decreasing cost industry has an upward sloping supply curve. E. None of these