Which subscription model innovation did Duolingo pioneer that changed how language learning apps monetize their services?

Language learning apps have transformed how we acquire new languages, using various subscription models to monetize their services. These platforms leverage linguistic research, machine learning, and behavioral psychology to create effective learning experiences. This trivia question tests your knowledge about how language learning platforms have innovated in the subscription economy to address different learner needs and behaviors.

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The 'streak maintenance' feature that allows users to pay to preserve their learning streak when they miss a day
The 'language rotation' model where users pay different rates depending on how many languages they're learning simultaneously
The 'vocabulary ownership' system where free users can only access words for 30 days unless they subscribe
The 'community translation' discount where users get subscription discounts for helping translate content