But we also know from all of the logic t hat we have developed before that if the firm is in the interior the solution is going to be given by marginal revenue = marginal cost And this is equal to p.In other words if the firm is going to choose the quantity of which marginal cost of that quantity equals to price, but that implies that the firm has to be there.Now what happens when the price is exactly the minimum ATC, while the firm is indifferent between choosing that point and that point, both of which give it a profit of zero and in red I have done what is the supply of this firm. 