Which of the assumptions in the theory of perfect competition assures us that economic profit will be zero in the long run?

Question

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The assumption of easy entry and exit assures us that economic profit will be zero in the long run. When there are profits to be made in a market, new firms will enter, increasing supply and driving down prices until profits are reduced to zero. Conversely, when there are losses to be incurred, firms will exit the market, reducing supply and driving up prices until losses are reduced to zero. This process continues until firms earn only a normal rate of return and economic profit is eliminated.

Which of the Assumptions in the Theory of Perfect Competition