Which product innovation introduced the 'spaced repetition system' (SRS) to mainstream language learning apps, revolutionizing vocabulary retention?

Product innovation in the language learning industry has evolved dramatically over the past decade. From traditional textbooks to AI-powered platforms, companies have developed increasingly sophisticated tools to help people acquire new languages. This poll tests your knowledge of a groundbreaking innovation that changed how millions approach language learning.

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Duolingo's 'crown levels' progression system
Anki's digital flashcard algorithm
Rosetta Stone's immersion-based interface
Babbel's speech recognition technology