【单选题】Which of the following best explains why a monopolist's marginal revenue is less than the sale price?
A.
To sell more units, a monopolist must increase the price on all units sold.
B.
As a monopolist expands output, its average total cost declines.
C.
When a firm has a monopoly, consumers have no choice other than to pay the price set by the monopolist.
D.
When a monopolist reduces price in order to sell more units, it must lower the price of some units that could otherwise have been sold at a higher price.