Why is washington, d. c. included in the southeast region?

There are no fixed regions available to make Washington D.C. in another region. It is where it should belong and if there was a law to change the location (if possible), then you could change the location. (in the northeast, midwest, etc.)
Hope this helps! :)

Cultural convergence is the theory that two cultures will be more and more like each other a very notable example of this is around the u.s.-mexico border, where 

Dall of the above all of the choices make sense