Which enable an oligopoly to form within a market?

i need more support for your question. thankyou

The correct answer is the costs of starting a competing business are too high.

What helps enabling an oligopoly to form within a market is the costs of starting a competing business are too high.

There are three basic conditions that are present when forming an oligopoly. The first one is that an oligopoly has few large Corporations. Secondly, in a monopoly, there is only one corporation. In an oligopoly market, there are high barriers to entry. The costs of starting a competing business are too high. And finally, the products oligopolistic corporations produce are differentiated of homogenous.  

b.europe i believe because north america has 3 countries in a way and with all the stuff with illegal immagrants from mexico or whatever so i would say europe.

i believe the answer is d.