Which subscription model innovation by Duolingo in 2017 dramatically changed their revenue stream while maintaining their 'language learning should be free' mission?

In the rapidly evolving field of language technology, companies are continuously innovating to meet market demands. Digital transformation has revolutionized how language learning platforms engage with users and monetize their services. This poll tests your knowledge about a significant product strategy shift in the language learning industry that changed how millions access linguistic education.

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Duolingo Plus (now Super Duolingo) with ad removal and offline lessons, keeping core learning free
Tiered pricing based on language difficulty, with common languages free and rare languages premium-only
Time-limited free trial model with automatic conversion to paid subscription after 30 days
Certification-only model where learning is free but official language proficiency certificates require payment