An opportunity cost is what was sacrificed to do or acquire something else. The condition of scarcity creates opportunity cost. If there was no scarcity, there would be no need to sacrifice one thing to acquire another. The opportunity cost would be much higher in New York City as the alternative uses for that square block are much more valuable than for a typical suburban city block.
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Economics is the study of scarcity and the condition of this creates opportunity cost. So, if there is no scarcity the there is nothing to sacrifice. The answers is the New York city because a central city would much more to give up than a suburban city.