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18.Which statement is true? a.The longrun industry supply curve in a competitive constantcost industry is perfectly elastic at the minimum of long-run average cost. b.The longrun industry supply curve in a competitive constantcost industry is perfectly elastic at the minimum of average variable cost. c.The long-run industry supply curve in a competitive constant-cost industry is perfectly elastic at the minimum of longrun marginal cost d.The long-run industry supply curve in a competitive constant-cost industry is long-run marginal cost curve above the minimum of longrun average cost e.none