When hiring workers, it is only profit maximizing for firms to hire additional workers when

a. The marginal benefit of each worker hired is positive. 

b. The marginal cost of each worker hired is above zero. 

c. The marginal benefit of each worker hired is less than the marginal cost of each worker hired.

d. The marginal benefit of each worker hired is greater than the marginal cost of each worker hired.

e. Each worker hired is productive.