Which innovative subscription model feature first popularized by Duolingo significantly changed the language learning industry's monetization approach?

The language learning industry has undergone a massive transformation with the rise of subscription-based services. These platforms have revolutionized how people acquire new languages, moving away from traditional one-time purchase models to recurring subscription plans that offer continuous updates and expanding content libraries. But how much do you know about the business models that power today's most successful language learning platforms?

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The freemium model with streak maintenance as a premium feature
Pay-per-word translation system
Grammar correction tokens that can be purchased individually
AI conversation partner access charged by the minute