Suppose Campus Books, a Profit-Maximizing Firm, Is the Only Supplier

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 below shows the reservation prices of the eight students enrolled in the class. $$\begin{array} { | c | c | } \hline \text { Student } & \begin{array} { c } \text { Reservation Price } \\\text { (\$/Book) }\end{array} \\\hline \mathrm { Q } & 60 \\\hline \mathrm { R } & 54 \\\hline \mathrm { S } & 48 \\\hline \mathrm { T } & 42 \\\hline \mathrm { U } & 36 \\\hline \mathrm { V } & 30 \\\hline \mathrm { W } & 24 \\\hline \mathrm { X } & 18 \\\hline\end{array}$$ What is the socially optimal number of books?

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