Which innovative subscription model feature did Duolingo introduce in 2017 that dramatically increased their revenue while maintaining their 'forever free' promise?

The language learning software market has exploded in the past decade, with different companies taking unique approaches to subscription models and user retention. Behind the colorful interfaces and gamified lessons are sophisticated business strategies that have transformed how we learn languages. Test your knowledge about one of the most successful language learning platforms and its subscription evolution!

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Duolingo Plus (now Super Duolingo) with offline lessons and no ads
Language certification tests that users could pay to take
Premium conversation partners with native speakers
Business team packages for corporate language training