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In 1981 he acquired a doctorate degree with his thesis Die deutschen Kriegsgefangenen in der Sowjetunion und das Nationalkomitee " Freies Deutschland " ( German prisoners of war in the Soviet Union and the National Committee for a Free Germany ) .
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Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
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