Suppose Campus Books, a profit-maximizing firm, is the only supplier of the textbook for a given class. The marginal cost of supplying each book is constant and equal to $10, and Campus Books has no fixed costs. The table shows the reservation prices of the eight students enrolled in the class.  CustomerReservation Price($/Book)Q60R54S48T42U36V30W24X18If Campus Books is permitted to charge 2 prices, and the bookstore knows customers with a reservation price above $30 never bother with coupons, whereas those with a reservation price of $30 or less always use them, then how many in total books will the bookstore sell?

A. 7
B. 6
C. 5
D. 8

Answer: A