Which language learning product development innovation first introduced the concept of 'spaced repetition' algorithms to optimize vocabulary retention?

The development of language learning products has evolved dramatically over the decades, with technological advancements creating new opportunities for linguistics education. Before language learning apps became ubiquitous, several innovative products transformed how people approached foreign languages. Test your knowledge about a significant milestone in language product development history that changed the industry forever.

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Rosetta Stone's immersion method (1992)
SuperMemo flashcard system by Piotr Wozniak (1985)
The Pimsleur Method audio courses (1967)
Duolingo's gamified learning platform (2011)