Which city is the headquarters of the Nuclear Threat Initiative (NTI)?

a) Washington, D.C., USA
b) Vienna, Austria
c) Geneva, Switzerland
d) Paris, France

The Nuclear Threat Initiative, generally referred to as NTI, is a non-profit organization located in Washington, D.C. The American foreign policy think tank was founded in 2001 by former U.S. Senator Sam Nunn and philanthropist Ted Turner and describes itself as a "nonprofit, nonpartisan global security organization focused on reducing nuclear and biological threats imperiling humanity.