Which unique customer retention feature did Duolingo introduce in 2018 that gamified language learning and became a case study in digital product loyalty?

Language learning apps have revolutionized how we approach linguistics education, with some brands creating innovative retail and customer experience models. This poll explores how Duolingo, one of the most successful language learning platforms, has pioneered unique customer retention strategies through its product design and loyalty features. Test your knowledge about this linguistic retail phenomenon!

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The Streak Society - a tiered membership program offering exclusive content to users who maintained learning streaks of 365+ days
Duolingo Plus Subscription - the premium paid tier that removed ads and offered offline learning
The Streak Freeze - allowing users to purchase an item that preserves their learning streak during a day of inactivity
Lingot Marketplace - a virtual currency exchange where users could trade earned points for physical Duolingo merchandise