When a oligopoly exists, how many producers dominate the market

In an oligopoly type of market, there are only few producers whom dominate the market. This is the market type wherein there are only small number of firms that control the majority of the market share. The opposite to Perfect Competition, wherein there are unlimited number of producers. In an oligopoly, since there are only a few numbers competitors, each firm is likely to be aware of each other's action.

Hydrosphere is the correct answer