单选题 The law of diminishing marginal returns explains: A. the shape of the long-run average cost curves. B. the upward sloping portion of the short-run marginal cost curve. C. the initial decrease in the short-run average costs.

【正确答案】 B

【答案解析】The law of diminishing returns states that at some point, as more of a resource is used in a production process, holding other inputs constant, output increases at a decreasing rate. This accounts for the upward slope of the SRMC curve beyond that point. Returns to scale determine the shape of the long-run cost curves.