Which customer feedback innovation revolutionized language learning product development in the 2010s?

The world of language learning products has evolved dramatically over the decades, with various brands employing innovative retail strategies and customer feedback mechanisms to refine their offerings. This poll tests your knowledge about a significant milestone in language learning product development that changed how customer feedback shaped the industry. Can you identify the groundbreaking approach that revolutionized language learning products?

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Duolingo's gamification based on real-time user progression data
Rosetta Stone's annual customer satisfaction mail-in surveys
Babbel's quarterly focus groups with language professors
Pimsleur's user experience tracking through cassette tape returns