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To lay an intellectual foundation for this argument , he cites two influential historians from the 20th century : Charles A. Beard and William Appleman Williams .
 Charles A. Beard, William Appleman Williams
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Charles A. Beard, William Appleman Williams