In order for a taxicab to be operated in New York City, it must have a medallion attached to its hood. Medallions are expensive, but they can be resold. To be a cabbie in London, an applicant needs to pass a test called "The Knowledge," in which one has to "master no fewer than 320 basic routes, all of the 25,000 streets that are scattered within the basic routes and approximately 20,000 landmarks and places of public interest that are located within a six-mile radius of Charing Cross." The NYC medallion, and the London "Knowledge" test, are examples of which type of cost: variable cost, fixed cost, sunk cost or opportunity cost? (Please indicate for each city's drivers the one most applicable type of cost and briefly explain why.)