Which major strategic pricing change did Duolingo implement in 2017 that dramatically altered its business model and user engagement strategy?

Duolingo has become one of the most recognizable language learning platforms globally, known for its gamified approach to language acquisition and its iconic green owl mascot. The company has revolutionized how people approach learning new languages through digital tools. This trivia question explores a pivotal moment in Duolingo's brand history that helped shape its current market position and customer loyalty strategy.

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Switched from a subscription-only model to a freemium approach with optional Duolingo Plus
Introduced a pay-per-lesson structure that charged users based on language difficulty levels
Removed all free content and transitioned to a premium-only platform with tiered pricing
Implemented a corporate partnership model where employers paid for employee language learning