What barriers to entry can create a monopoly?

• A. Low start-up costs, many close substitutes, and free access to distribution networks
• B. Temporary advertising advantages, occasional price leadership, and short-term demand spikes
• C. Government-imposed price ceilings, open-source technology access, and many small independent firms
• D. Legal protections (patents and exclusive licenses), control of essential inputs, and large economies of scale that deter rivals

Answer: (D)