Which innovative subscription model did Duolingo pioneer that revolutionized the language learning app market and has since been adopted by many competitors?

The language learning app market has exploded in the last decade with various subscription models designed to maximize both user engagement and revenue. Companies have experimented with different approaches to pricing their language learning tools, from freemium to tiered subscriptions, with fascinating results. This poll tests your knowledge about a revolutionary pricing strategy that changed how we approach language learning subscriptions.

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The 'streak maintenance' model, where users pay a small fee to maintain learning streaks when they miss days
The 'language bundling' approach, where users pay more to access premium languages like Mandarin or Arabic
The 'grammar escalation' model, where basic grammar is free but advanced constructions require payment
The 'vocabulary threshold' system, where users hit paywalls after learning a certain number of words in each language