Answer:
Difficult entry, Market control by a few large firms, Mutual interdependence
Explanation:
An Oligopolistic market is recognized because and small group of companies controls an specific market segment. therefore, when a new company tries to enter to the market there are barriers established by the existing businesses in the same segment, sometimes these are price, supplies and marketing barriers.
No matter the size of the companies, there are not many competitors in the specific segment that they share. so, these companies that are part of the oligopolistic market controls it, if they change something they affect to the other members of the oligopolistic market, therefore they have mutual interdependence characteristics. in many models like this the competitor work together avoiding to cause negative effects in the other companies. as an example when there is a duopolistic market, if there is a leader, he can define the prices and the other company may select the production units.