Which of the following is not an example of barriers to entry? a. cable companies must lay miles of underground cable before they can serve a single customer in a new market. b. in some counties, laws require retail stores to be closed on sundays. c. an entrepreneur wishing to own a clothing store must rent a building, hire workers, and buy clothing for sale. d. a person who wishes to practice medicine is required to attend medical school, complete an internship, and pass a state exam.