Q. Which country built the world's first nuclear reactor?
Answer: USA
Notes: The United States built the world's first nuclear reactor, Chicago Pile-1, achieving criticality on December 2, 1942 in Chicago under the Manhattan Project. The reactor was designed by Enrico Fermi and operated at the University of Chicago. The USSR constructed subsequent reactors, including the Obninsk Nuclear Power Plant in 1954, but Chicago Pile-1 was the first reactor ever brought to criticality.