Fiona Has a Monopoly on Motorboat Rentals on Nantucket Island

Fiona has a monopoly on motorboat rentals on Nantucket Island during the summer. She can rent five boats per week at $21,000 each. If she wants to rent six, she can charge only $20,000 each. The quantity effect of renting the sixth motorboat is:

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