The Washington Nationals are a professional baseball team that has been based in Washington, D.C. since . The Nationals are a member of both the Major League Baseball's (MLB) National League Eastern Division and the National League (NL) itself. Since the 2008 season, the Nationals have played in Nationals Park; from 2005 through , the team played in Robert F. Kennedy Memorial Stadium.
The Washington Nationals has always been based in Washington D. C.
The context says "The Washington Nationals are a professional baseball team that has been based in Washington, D.C. since "  This statement indicates that there may or may not be  a period where the team was not based in D. C.  It was probably difficult for the system as there is no info on when the team was created, or when it started to be based in DC