A Reason Why a Perfectly Competitive Firm's Demand for Labor

A reason why a perfectly competitive firm's demand for labor curve slopes downward is that

A) each additional unit of labor hired is less efficient than previously hired units.
B) in the short run, as more labor is hired, labor's marginal product falls because of the law of diminishing returns.
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.