A perfectly competitive industry is said to be efficient because the:

A) marginal cost of production of the last unit of output is minimized in the long run.
B) product is standardized across firms in the industry.
C) average total cost of production of the industry's output is minimized in the long run.
D) market price of the good is equal to economic profit for all firms in the industry.

In a perfectly competitive industry, the firms have no market power, so they must accept the market price for their output. This market price is determined by the intersection of the industry's supply and demand curves. In the long run, firms in the industry adjust their production levels and inputs to minimize their average total cost of production. This leads to a situation where the industry produces the desired quantity of output at the lowest possible cost. Therefore, the average total cost of production of the industry's output is minimized in the long run, making it efficient.