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Severin Hacker Severino Zuazo Severina Mana'o Severino Poletto Severin, Ludwigslust-Parchim Severinus Séverine Nébié Séverine Caneele Severnaya Zemlya SeveriaTài liệu tham khảo
WikiPedia: Severin Hacker https://techcrunch.com/2011/04/12/duolingo/ https://www.forbes.com/sites/tomiogeron/2012/09/17... https://www.technologyreview.com/innovator/severin... https://www.onlinecoursereport.com/duolingo-the-mo... https://csq.com/2020/02/severin-hacker-duolingo-in... https://mashable.com/archive/learn-a-new-language-... https://www.cs.cmu.edu/news/2014/crunchie-award http://nextpittsburgh.com/business-tech-news/pitts... https://www.oneyoungworld.com/news-item/one-young-... https://www.startupticker.ch/en/news/june-2015/not...