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The Late-May 2008 tornado outbreak sequence from May 22 , 2008 to May 31 , 2008 affected much of the central United States and parts of Canada .
 United States, Canada
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 United States, Canada