1776年,美国建国。至今200多年的时间里,美国发生了翻天覆地的变化。从地理上讲,美国从大西洋沿岸一个拥有13州、80多万平方公里的狭长地带扩张到横跨北美大陆、拥有50个州和众多海外领地的世界第四大国;从人口上讲,从人种相对单一的300多万人口增加到今天几乎包含世界所有种族和族裔的3.2亿人;从经济上来讲,从经济总量在世界占比可以忽略不计发展到经济总量 In 1776, the United States founded. Over the past 200 years or so, the world has undergone tremendous changes. Geographically speaking, the United States has expanded from a narrow strip of more than 800,000 square kilometers in a state with 13 states on the Atlantic coast to the fourth largest country in the world with 50 states and numerous overseas territories across the North American continent. From a population perspective, The relatively single 300 million population has increased to almost 320 million today, covering all ethnicities and ethnic groups in the world. Economically speaking, from a negligible proportion of the total economic output in the world to total economic output