In a Labor Market with an Effective Union,the Marginal Revenue

In a labor market with an effective union,the marginal revenue curve lies below the demand curve because:

A) workers who are part of the union are not as productive as non-union workers.
B) union workers are paid less than their marginal revenue product.
C) the union must lower wages paid to all workers to hire an extra worker.
D) marginal revenue increases as additional workers are hired.