Concept:A firm expands production as long as the extra revenue from an additional unit exceeds its extra cost.Explanation:• If MC<MR, the marginal cost of producing one more unit is less than the marginal revenue it brings.• Therefore, the perfect competitor can increase profit by expanding output.• The firm stops expanding only when MC=MR, but among the given options, it will continue expanding while MC<MR.Answer:C. MC<MR