【单选题】If a monopsonist pays the wage rate w; then the amount of labor that he can hire is L(w) = Aw, where A is a positive constant. The marginal cost of labor to the monopsonist is:
A.
equal to the wage rate.
B.
twice the wage rate.
C.
half the wage rate.
D.
greater than the wage rate but less than twice as great.