File photo taken on April 17, 2014 shows tourists watch tulips on Capitol Hill in Washington D.C., capital of the United States. Named in honor of first American President George Washington, Washington D.C. or Washington District of Columbia is the capital of the United States that is located between Maryland and Virginia states. The U.S. Constitution provided for a federal district under the exclusive jurisdiction of the Congress and the District is therefore not a part of any U.S. state. Washington D.C. has an estimated population of 660,000. Washington D.C. and Beijing became sister cities in 1984. (Photo: Xinhua/Yin Bogu)
