Given the following context:

The Smithsonian Institution is an educational foundation chartered by Congress in 1846 that maintains most of the nation's official museums and galleries in Washington, D.C. The U.S. government partially funds the Smithsonian and its collections are open to the public free of charge. The Smithsonian's locations had a combined total of 30 million visits in 2013. The most visited museum is the National Museum of Natural History on the National Mall. Other Smithsonian Institution museums and galleries on the mall are: the National Air and Space Museum; the National Museum of African Art; the National Museum of American History; the National Museum of the American Indian; the Sackler and Freer galleries, which both focus on Asian art and culture; the Hirshhorn Museum and Sculpture Garden; the Arts and Industries Building; the S. Dillon Ripley Center; and the Smithsonian Institution Building (also known as "The Castle"), which serves as the institution's headquarters.
The Smithsonian American Art Museum and the National Portrait Gallery are housed in the Old Patent Office Building, near Washington's Chinatown. The Renwick Gallery is officially part of the Smithsonian American Art Museum but is in a separate building near the White House. Other Smithsonian museums and galleries include: the Anacostia Community Museum in Southeast Washington; the National Postal Museum near Union Station; and the National Zoo in Woodley Park.
The National Gallery of Art is on the National Mall near the Capitol and features works of American and European art. The gallery and its collections are owned by the U.S. government but are not a part of the Smithsonian Institution. The National Building Museum, which occupies the former Pension Building near Judiciary Square, was chartered by Congress and hosts exhibits on architecture, urban planning, and design.There are many private art museums in the District of Columbia, which house major collections and exhibits open to the public such as the National Museum of Women in the Arts and The...

answer the following question:

What is the acronym for the United States? 

U.S.