Which language learning product innovation was the first to implement adaptive spaced repetition algorithms at scale, revolutionizing vocabulary acquisition?

The language learning and linguistics industry has undergone remarkable transformations in recent decades, with technology playing an increasingly central role. Product development in this space has created tools that would have seemed like science fiction just a generation ago. This trivia question focuses on a pivotal product development milestone that changed how people approach language learning worldwide. Test your knowledge of language technology product history!

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SuperMemo (1987)
Rosetta Stone's Dynamic Immersion (1992)
Pimsleur's Audio Method (1967)
Duolingo's Gamification System (2011)