If a Firm Can Increase Output by Hiring More Workers

If a firm can increase output by hiring more workers then

A) it will always do so.
B) it will never do so.
C) it will do so only if the cost of hiring the workers (and purchasing the materials) is less than the increase in revenues associated with the increase in sales.
D) it will do so only if the cost of hiring the workers (and purchasing the materials) is more than the increase in revenues associated with the increase in sales.