Which marketing strategy did Duolingo famously pioneer that revolutionized language learning app monetization?

Test your knowledge about innovative marketing strategies used by language learning and translation technology companies! The language tech industry has developed unique approaches to customer acquisition and retention. From freemium models to strategic partnerships with educational institutions, these companies have revolutionized how language tools are marketed. How well do you understand their successful strategies?

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The 'hearts' system limiting daily lessons unless users subscribe
Celebrity endorsements featuring polyglots and linguists
Exclusive university partnerships requiring student subscriptions
Pay-per-word translation model with tiered pricing