Answer:
A sound firm hires input(workers) in as much as marginal benefit from it is greater than marginal cost of it. The hiring is done till both become equal(MB=MC). Since Marginal Product Labor is always positive, total revenue will rise.
However, Firms do not continuously hire new workers because of the reason below:
a. Marginal cost exists and is positive. When value of marginal product of labor becomes less than marginal cost (Wage), it is not profitable to hire more labor. This is loss making.
b. Hiring is done till VMPL > Wage.