A reason why a perfectly competitive firm's demand for labor curve slopes downward is that:
a. In the short run, as more labor is hired, labor's marginal product falls because of the law of diminishing returns.
b. Each additional unit of labor hired is less efficient than previously hired units.
c. The extra cost of hiring additional units of labor increases as a firm hires more units of labor.
d. The firm's demand curve for the product that uses labor is downward sloping.