Please help with this practice question. This is the full question.

Which of the following is not an example of a barrier to entry that helps a firm maintain its monopoly power? 0 Owning the only basketball arena in Oakland. 0 A patent giving a pharmaceutical company exclusive rights to produce a drug for 20 years. 0 Grocery stores only selling products from well-known distributors. O A law restricting alcohol sales past 2 AM in California. 0 Requiring all doctors to be licensed