The most frequently found barriers to entry in oligopoly include

A. The presence of large numbers of rival firms, firms that are price takers, and the likelihood of normal returns in the long run.

B. Inability to differentiate product, diseconomies of scale, and the need to advertise.

C. Patents, large initial and continuing financial requirements, and access to basic input.

D. Mutual interdependence, collusion, and the likelihood of government regulation.

E. The ambiguity of product groups and rising long-run average costs.