Which product design innovation first successfully incorporated 'spaced repetition algorithms' into digital language learning tools, revolutionizing vocabulary retention?

In the intersection of linguistics and product design, innovative approaches have transformed how we interact with language learning tools. Modern language acquisition products incorporate sophisticated design principles to enhance user experience and learning outcomes. This poll tests your knowledge about a groundbreaking design innovation that revolutionized how users interact with language learning interfaces.

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SuperMemo, which pioneered the SM-2 algorithm in 1987 that formed the foundation for modern spaced repetition software
Rosetta Stone's Dynamic Immersion methodology, first implemented in their 1992 CD-ROM product
Duolingo's gamified reinforcement system, introduced with their mobile app in 2012
Babbel's contextual learning framework, developed for their first online courses in 2008