Sally's Salon Sells Haircuts in a Perfectly Competitive Market

Sally's Salon sells haircuts in a perfectly competitive market.As the salon hires additional workers,the marginal revenue product of labor

A) rises whenever the marginal product of labor falls
B) is equal to the market price of haircuts
C) falls whenever the marginal product of labor falls
D) varies only if the market price of haircuts varies
E) remains constant