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This period in Prussia was marked by a persecution of Old Lutherans who refused to use join the Prussian Union , under King Frederick Wilhelm III .
 Prussia, Prussian Union, Frederick Wilhelm III
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Prussia, Prussian Union, Frederick Wilhelm III