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They were among the most popular Serbian and Yugoslav clubs , and they were nicknamed as " Crveni " ( The Reds ) because of their red shirts , in opposition to their greatest rivals BSK , who wore blue and were known as " Plavi " ( The Blues ) .
 Yugoslav, BSK
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Yugoslav, BSK