Which innovative product feature was first introduced by Duolingo in 2016 that revolutionized language learning apps by addressing the 'forgetting curve' problem?

In recent years, language learning apps have transformed how people acquire new languages, with various innovative product features competing for user engagement. One breakthrough feature has particularly revolutionized the industry by addressing a long-standing challenge in language acquisition. Test your knowledge about product innovations in language learning technology that have shaped the modern linguistic education landscape.

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Spaced Repetition System (SRS)
Immersive Conversation AI
Peer Language Exchange Network
Augmented Reality Vocabulary Scanner