An Individual Firm's Demand for a Factor of Production

An individual firm's demand for a factor of production

A) slopes downward because an increase in the production of output reduces the price at which the output can be sold in a competitive market, thereby reducing the value of the marginal product as more of the factor is used.
B) slopes downward due to the factor's diminishing marginal product.
C) slopes upward due to the factor's increasing marginal product.
D) is perfectly elastic (horizontal) if the factor market is perfectly competitive.