The International Nickel Company of Canada is often cited as an example of monopoly. What was the source of the barrier to entry that gave this firm monopoly​ power? A. control of a key resource B. There were important network externalities in the production of nickel. C. It was a public​ enterprise; therefore, the Canadian government blocked entry into the market for nickel. D. Economies of scale resulted in the company becoming a natural monopoly.