A firm's average cost increases as it increases its output by expanding its plant and hiring additional workers (its only inputs to production). The firm's owner blames the increase in per-unit costs on the law of diminishing marginal productivity. The owner's reasoning is: A. correct because some inputs are fixed in the long run. B. incorrect because economies of scale are present. C. correct because marginal productivity must decrease in the short run. D. incorrect because all inputs are varied in the example.

Answer: D. incorrect because all inputs are varied in the example.

Explanation: While marginal productivity describes the extra output, or return, or profit gotten per unit by benefits from the production inputs of a company, the law of diminishing marginal productivity is one that recognizes that the quantity of all inputs of production cannot be changed at one time. The owner's reasoning of attributing the increase in per-unit costs on the law of diminishing marginal productivity is incorrect because all inputs are varied in the example. Marginal productivity eventually declines because some inputs are fixed, but however, in the long run where no inputs are fixed, the law does not apply.