The correct answer to this question is that:
In a monopoly, “the monopolist
must lower the price on all units to sell one more unit of output”.
This means that in a monopoly market,
if we increase the amount of output without lowering the price, the marginal
revenue decreases. Therefore marginal revenue is indirectly proportional to
number of outputs.
In a perfect competition however, the
marginal revenue is constant to any amount of output.
