Which innovative customer feedback analysis technique revolutionized Duolingo's language learning algorithm in 2019?

In the competitive world of language learning apps and platforms, customer feedback analytics have transformed how products evolve. Modern linguistics-focused tech companies employ sophisticated methods to analyze user satisfaction and learning outcomes. This poll tests your knowledge about how linguistic product developers use feedback data to enhance their offerings and create more effective language learning experiences.

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Sentiment-mapped progression tracking that analyzed emotional responses to correctly/incorrectly answered questions
Voice-pattern recognition that measured user frustration levels during speaking exercises
Eye-tracking analysis that monitored user engagement with different linguistic elements
Social learning metrics that evaluated collaborative problem-solving among language learners