To the OP, maral55:
Everyone in the thread did it instinctively, but it should be noted because it is a very common mistake.
Remember: You must use the article the with every form mentioned except America. Many learners forget this, especially when using the abbreviations.
It's the US and the USA, never just US and USA (as long as they're not modifying a noun, of course).
Thanks to all of you friends. That was very enlightening, especially to have the native speakers opinions. I really appreciate your help.