Which language learning platform pioneered the 'freemium' subscription model with a 'streak' feature that significantly boosted user retention and dramatically increased conversion to paid subscriptions?

The language learning app market has exploded in recent years, with subscription models driving tremendous revenue growth. Major players have revolutionized how people learn languages through innovative pricing strategies, engagement mechanics, and retention approaches. This poll tests your knowledge about the business models behind these linguistic learning platforms and how they've achieved market dominance through strategic subscription offerings.

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Duolingo, which saw a 71% increase in paid subscribers after implementing the streak feature that gamified daily practice
Babbel, which introduced the first linguistically-adaptive subscription tiers that adjust pricing based on language difficulty
Rosetta Stone, which created the first 'lifetime access' subscription model with embedded AI conversation partners
Memrise, which developed a location-based subscription model that unlocks premium content when users travel to countries speaking their target language