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参考答案:The United States of America, also known as the United States, the U.S., the U.S.A., and America, is a country in North America. A federal republic, the United States shares borders with Canada and Mexico, and extends from the Atlantic Ocean to the Pacific Ocean.The United States is also in close proximity to Russia via the state of Alaska; separated only by the Bering Strait. Its capital is Washington, D.C.
The present-day continental United States has been inhabited for at least 15,000 years by indigenous tribes.[1] After European exploration and settlement in the 16th century, the English established their own colonies, and gained control of others that had been begun by other European nations, in the eastern portion of the continent in the 17th and early 18th centuries. On 4 July 1776, at war with Britain over fair governance, thirteen of these colonies declared their independence; in 1783, the war ended in British acceptance of the new nation. Since then, the country has more than quadrupled in size: it now consists of 50 states and one federal district, and has a number of overseas territories.
At over 3.7 million square miles (over 9.5 million km²), the U.S. is the third or fourth largest country by total area, depending on whether or not the disputed areas of China are included. It is also the world's third most populous nation, with nearly 300 million people.
The United States has maintained a liberal democratic political system since it adopted its Articles of Confederation on 1 March 1781 and the Constitution, the Articles' replacement, on 17 September 1787. American military, economic, cultural, and political influence increased throughout the 20th century; with the collapse of the Soviet Union at the end of the Cold War, the nation emerged as the world's sole remaining superpower.[2] Today it plays a major role in world affairs.