For a Single-Price Monopolist, Marginal Revenue Is Less Than Price

For a single-price monopolist, marginal revenue is less than price because

A) the revenue gain from the last unit sold is offset by a revenue loss on the units that previously had been sold at a higher price.
B) the revenue gain from the last unit sold is offset by further gains in price on units not sold at all.
C) total revenue always decreases as output increases.
D) the price does not have to be lowered on all previous units sold.