Which innovative subscription model feature first introduced by Duolingo in 2017 became a game-changer for the language learning industry and was later adopted by competitors?

The language learning industry has been transformed by subscription-based models in recent years. These platforms have revolutionized how people access linguistic content and develop language skills. Test your knowledge about one of the most significant business model shifts in the language learning landscape and see how well you understand the economics behind modern language acquisition tools.

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Streak Society (premium tier based on consecutive days of practice)
Super Duolingo (formerly Duolingo Plus) with offline lessons and no ads
Crowdsourced course creation system with volunteer contributors
Stories feature with interactive narrative comprehension exercises