The Textbook for Your Class Was Not Produced in a Perfectly

The textbook for your class was not produced in a perfectly competitive industry because:

A) there are so few firms in the industry that market shares are not small, and firms' decisions have an impact on market price.
B) upper-division microeconomics texts are not all alike.
C) it is not costless to enter or exit the textbook industry.
D) of all of the above reasons.