According to the cost-benefit principle, under what conditions should a business owner hire an addition worker?
a. when the total benefit of all the workers will be greater than or equal to the total cost of all the workers
b. when the total cost of all the workers will be greater than or equal to the total benefit of all the workers
c. when marginal cost is greater than or equal to marginal benefit
d. when marginal benefit is greater than or equal to marginal cost