参考答案：C

解析：

In the short run, the average product of labor curve is first increasing and then decreasing as diminishing marginal returns to that factor take effect. In the short run, the marginal product of labor is first increasing and then decreasing when diminishing marginal returns take effect. The marginal product of labor curve will be above the average product of labor curve initially, and, at some point, will intersect the average product curve at its maximum. When the total product of labor begins to increase at a decreasing rate, the average product of labor will be decreasing.