Which example below represents an oligopoly?

-A farmergrows corn that is identical to all of the other corn grown in that region.

-In the country where this company is located, government regulations prevent competition from other companies.

-A company produces jeans that are pretty much the same as other jeans on the market, except for their advertising, which features a world-famous supermodel.

-In this industry, groups of firms work together to establish stable pricing.