In order to maximize profit, the firm should hire workers and buy capital until

• a. the marginal product per dollar of labor equals the marginal product per dollar of capital
• b. the wage of labor equals the cost of capital
• c. the marginal revenue product per dollar cost is equal for labor and capital
• d. the marginal product of labor equals the marginal product of capital
• e. the number of workers equals the number of pieces of capital