Select two different industries. Determine what market structure firms within these industries find themselves in. What role does the ease of entry and exit play in the classification of different market structures? What role does presence of substitutes play? Can an industry evolve from perfect competition to monopolistic competition to oligopoly to monopoly? Give an example? What about the other way? Can an industry go from monopoly to oligopoly to monopolistic competition to perfect competition? Provide examples, defend your answers and cite your sources.