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

The language learning industry has seen remarkable product innovations over the decades, transforming how people acquire new languages. From traditional textbooks to interactive digital platforms, technological advancements have revolutionized this field. This trivia question focuses on a particular product innovation that disrupted the language learning market and created a new paradigm in linguistic education. Test your knowledge about the history of language learning product development!

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SuperMemo (1987), which pioneered the SM-2 algorithm that calculated optimal intervals for reviewing flashcards based on user performance
Rosetta Stone (1992), which introduced computer-based immersive learning through image association rather than translation
Pimsleur Language Programs (1963), which created the graduated interval recall method for audio language courses
Duolingo (2011), which developed the first adaptive learning algorithm that personalized language exercises based on user progress