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In the 9th and 10th centuries , Christianity made great inroads into pagan Europe , including Bulgaria ( 864 ) and later Kievan Rus ' ( 988 ) .
 Europe, Bulgaria, Kievan Rus '
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Europe, Bulgaria, Kievan Rus '