When Comparing Perfect Competition and Monopoly, a Major Assumption Made

When comparing perfect competition and monopoly, a major assumption made is that

A) the monopolist faces a downward sloping demand curve.
B) consumers only care about the price of the good and not whether the seller is a monopoly or not.
C) the costs of production are the same under monopoly as under perfect competition.
D) the monopolist can make an above normal rate of return.