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