Samantha Has a Monopoly in the Retail of Ski-Doos

Samantha has a monopoly in the retail of ski-doos. She can sell five units per week at a price of $21,000 each, but to sell her full weekly capacity of six units, she must charge $20,000 each. The quantity effect of selling the sixth ski-doo is:

A) $20,000.
B) $10,000.
C) $15,000.
D) $21,000.