Which innovative product development feature did Duolingo introduce in 2020 that dramatically changed its language learning experience?

Duolingo revolutionized language learning through its mobile app, making education accessible to millions worldwide. The company's product development journey offers fascinating insights into how language learning products evolve based on user behavior, cognitive science, and technological capabilities. Test your knowledge about this language learning platform's growth strategy and innovative features that changed how we approach linguistics education.

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Stories - interactive narratives with comprehension exercises
Duolingo Plus subscription model with offline learning
AI-powered speaking exercises with real-time pronunciation feedback
Competitive leaderboards and achievement badges system