A Competitive Firm Sells Its Output for $45 Per Unit

A competitive firm sells its output for $45 per unit.The marginal product of the 30th worker is 4 units of output per day;the marginal product of the 31st worker is 3 units of output per day.The firm pays its workers a wage of $150 per day.

A) For the 31st worker, the marginal profit is $135.
B) For the 30th worker, the value of the marginal product of labor is $600.
C) For the 30th worker, the marginal profit is $180.
D) None of the above is correct.