Which language learning product innovation first successfully implemented spaced repetition algorithms to optimize vocabulary retention in a commercial product?

In the world of language learning products, technological innovations have transformed how we acquire new languages. One breakthrough product revolutionized the industry by introducing a novel approach to language acquisition that challenged traditional methods. Test your knowledge about this significant product development milestone that changed how millions approach language learning worldwide.

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Rosetta Stone's Dynamic Immersion method (1992)
SuperMemo by Piotr Wozniak (1987)
Duolingo's gamification system (2011)
Pimsleur's graduated interval recall method (1967)