I want to say the Appalachian Mountains.
Explanation:
A developed country (or industrialized country, high-income country, more economically developed country (MEDC)) is a sovereign state that has a high quality of life, developed economy and advanced technological infrastructure relative to other less industrialized nations. Most commonly, the criteria for evaluating the degree of economic development are gross domestic product (GDP), gross national product (GNP), the per capita income, level of industrialization, amount of widespread infrastructure and general standard of living.
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