I am facing problem in autocad 2013 problam

hello! there is problem in my autocad 2013 when i open multipal files in one autocad session that,s opens new autocad session not in the same one autocad session... 2nd question is when i have to close the file from autocad 2013 that does not close so that i have to close full autocad software first then i reopen autocad 2013 for doing work on other file. please help me....

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