Using the Microeconomic Theory Basic Principles and Extensions, Nicholson, W. and Snyder, C., I've reached to the point where Monopoly and Imperfect Competition is discussed (Chapter 14 and 15 to be more specific).
It's kind of easy to deduce that Marginal Cost equals Marginal Revenue at the point of profit maximization. In the imperfect competition in the long-run (or in the perfect competition) no one can surpass the point where price equals marginal cost equals average cost, because above this point firms will have negative economic profit.
The main question is why (and not only the graphic explanation please) the firm will choose AC (average cost) = MC (marginal cost)?
Graphically, it looks like the slope of the Demand Curve makes this choice the best one, but why in imperfect competition is this choice not pursued by the firm instead of the Supply Curve?