Monopolistically competitive firms are unable to affect the market price of their output, but are able to control the price of their own output.

Suppose that a pure monopolist can sell 20 units of output at $10 per unit and 21 units at $9.75 per unit.The marginal revenue of the 21^st unit of output is:

A) $9.75.
B) $204.75.
C) $4.75.
D) $.25.