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

Multiple Choice

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.  
$$\begin{array} { | c | c | } \hline \text { Custamer } & \begin{array} { c } \text { Reservutian Price } \\\text { (S/Bank) }\end{array} \\\hline \mathrm { Q } & 60 \\\hline \mathrm { R } & 54 \\\hline \mathrm { S } & 48 \\\hline \mathrm { T } & 42 \\\hline \mathrm { U } & 36 \\\hline \mathrm { V } & 30 \\\hline \mathrm { W } & 24 \\\hline \mathrm { X } & 18 \\\hline\end{array}$$ What is the socially optimal number of books?

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