Which innovative feature, first introduced by Duolingo in 2016, revolutionized language learning apps by incorporating cognitive science principles?

Language learning apps have revolutionized how people acquire new languages. Behind these digital tools are fascinating product development stories and innovative approaches to language acquisition. This poll tests your knowledge about a significant product launch that changed the language learning landscape. Do you know which groundbreaking feature revolutionized the industry?

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Spaced Repetition System (SRS) that automatically adjusts review intervals based on user performance
Augmented Reality translation that overlays translated text when pointing camera at foreign language text
Neural voice recognition that provides accent-specific pronunciation feedback
Contextual grammar correction that explains rules based on specific user errors