Answer: Oligopoly
Explanation:
Oligopoly could be described as when a market structure has few firms where none of them have a greater influence than the other. This is having more than two firms having large influence and are not affected by the others presence.
Gilligan's Island having three large supermarkets that supply most of the groceries for the island's population refers to an Oligopoly, as the three firms all do well and are not influenced by the other.
