Asunción (Paraguay) is also 8 letters long. For Washington, D.C., I had included it once in my quiz, and Eutychus commented "The capital of the U.S. is Washington. The District of Columbia is the political entity it is part of- kind of like a state. DC is not part of the name."
Thanks for Paraguay. :) I live right by the D.C. area, and no one calls it Washington. Had someone referred to it such a thing I'd've mistaken it for the state!
It is called Washington D.C. on the World Capitals Quiz, which is made by QM, an American. The city mostly referred to Washington D.C. or D.C (which is short for Washington D.C.) and is never called just Washington.