Свопросами, заранее . the capital of the usa is washington, d.c.,isnt it? what do the letters d.c stand for? why do people say "washington, d.c" when they speek about the us capital?

1) Yes, it is.

2) To distinguish it with the state of Washington.

3) It means Washington, District Columbia. There's the state of Washington on the west coast. They do it to disitnguish them.