Which of the following statements is true in regarding to the marginal cost of labour?

a$.$

is derived from the marginal revenue product curve.

b$.$

shows the increase in revenue received by the firm if it hires one more worker.

c$.$

is the same as the supply curve of labour for a monopsonist.

d$.$

is equal to the wage paid if the firm is a monopsonist.

e$.$

is horizontal for a perfectly competitive firm but is downward sloping for a monopsonist.