For a Firm Selling Its Product in a Perfectly Competitive

For a firm selling its product in a perfectly competitive market, the demand curve for a factor of production is downward sloping because

A) a factor's marginal product increases when the price of the factor increases.
B) firms must hire more factors of production if they want to increase their output.
C) to sell more output, the firm must lower the price of its product.
D) of the diminishing marginal product of the factor.
E) None of the above answers is correct.