问题
单项选择题
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.
答案
参考答案：B
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.