A friend stated, "I don't understand why public museums charge entrance fees." I quickly put on my economist's hat and explained the logic of an entrance fee: public museums are examples of and are therefore subject to Entrance fees are a policy response to such market failures. Select one: O a. Public goods; free riding. O b. Open-access resources; the tragedy of the commons. O c. Open-access resources; free riding O d. Public goods; the tragedy of the commons