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===> What South-American country has the largest population?

Brazil has the highest population among countries in South America.

Brazil

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Population:
198,739,269
country comparison to the world: 6
note: Brazil conducted a census in August 2000, which reported a population of 169,872,855; that figure was about 3.8% lower than projections by the US Census Bureau, and is close to the implied underenumeration of 4.6% for the 1991 census (July 2009 est.)
Data for all countries

Source: The World Factbook 2009