Explain the source of monopoly power for DeBeers' Diamond Mine in South Africa, Microsoft (owned by Bill Gates), the American Medical Association (which licenses doctors), Polaroid's Instant Picture Cameras, USAir (which owns virtually all the gates at the airport in Charlotte, North Carolina), and electric utilities

DeBeers' controls the scarce resource by controlling the distribution of the vast majority of the world's diamonds. Microsoft has marketing superiority and perhaps a unique resource in Bill Gates, now a multibillionaire. The AMA restricts the supply of doctors through licensing (a legal restriction). Polaroid had an (expired) patent that allowed it to monopolize instant pictures. It successfully challenged Kodak's attempt to enter the instant photography market. USAir is using a deliberately erected entry barrier; planes cannot fly into or out of an airport without a gate. Electric companies have economies of scale (as well as large sunk costs).