The major difference is that; a perfectly competitive firm produces has the choice to dictate the number of products to make, whereas a monopoly system has market power.
Perfectly competitive producers are those who can determine how the number of products to be made but not the price at which their output may be sold.
However, a monopolistic system has the power to dictate the price because there is only one producer and numerous customers. Hence, they possess a high market power than the perfectly competitive producers.
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