Which subscription model innovation was FIRST introduced by Duolingo to revolutionize language learning monetization while maintaining free access?

Language learning platforms have revolutionized how we acquire new languages, with subscription models playing a crucial role in their business strategy. These platforms utilize various approaches to monetize their services while keeping users engaged in their language learning journey. This poll tests your knowledge about innovative subscription strategies in the language learning industry that have shaped customer engagement and retention.

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Hearts system limiting daily lessons for free users with unlimited hearts for subscribers
Tiered proficiency certifications where basic certifications are free but professional-grade ones require subscriptions
The 'language family' model where one language is free but related languages require subscription
AI conversation partners with free text chats but premium voice interaction for subscribers