What effect might the government have on oligopolies?

In oligopolies, the government might

A$.$ impose barriers to entry with a copyright, which allows only the government to supply a good or service.

B$.$ promote competition by allowing large firms to lobby state legislators and members of Congress for favorable laws.

C$.$ promote competition with a patent, which grants exclusive rights to product a good.

D$.$ impose barriers to entry with occupational licensing, where a license is required to provide a good or service.

E$.$ impose barriers to entry with a free$-$trade agreement with another country to promote competition.