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Most scholars writing at the end of the 19th century estimated that the pre-Columbian population was as low as 10 million ; by the end of the 20th century most scholars gravitated to a middle estimate of around 50 million , with some historians arguing for an estimate of 100 million or more .
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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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