為何美國國土華盛頓Washington後面要加DC?

right in the beginning, Washington is no place where it are now.
the new grovernment decided to take up some land from State of Maryland. and they call it a special district. name Columbia.
then the full name become-------- Washington, district of Columbia ( DC ).

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美國首都全寫是華盛頓，哥倫比亞特區，簡寫Washington, D.C.
哥倫比亞特區地位等同州，並和華盛頓州作區分。

美國係有一個 Washingson州，Washington D.C.係首都，是不同地方。