When Labour Is a Firm's Only Variable Input in Its

When labour is a firm's only variable input in its production process, what point will a profit-maximizing firm continue to employ additional workers until?

A) as long as the marginal revenue product of labour > the marginal resource cost
B) as long as the marginal product of labour = 0
C) as long as the marginal product of labour > 0
D) as long as the marginal revenue product of labour < the marginal resource cost