Which packaging innovation revolutionized language learning products in the early 2000s by introducing a 'spaced repetition system' directly on physical flashcards?

In the world of language learning products, packaging innovations have played a crucial role in enhancing user experience and product effectiveness. From traditional language dictionaries to modern language learning apps, the way linguistic products are 'packaged' has evolved dramatically. This poll tests your knowledge about a revolutionary packaging innovation that changed how language learning materials were presented to consumers in the digital era.

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Leitner BoxSystem
SRS Digital Packaging
SuperMemo Physical Cards
Pimsleur Graduated Interval Recall Packaging