Which customer feedback innovation revolutionized mathematics textbook development in the 2010s, leading to the industry's current product strategy?

The mathematics educational materials market has undergone significant transformations in recent years. Publishers have adapted to digital learning environments, responded to curriculum changes, and faced new competition from innovative startups. This poll tests your knowledge about a pivotal customer feedback-driven change that transformed how mathematics textbooks are developed and marketed to educational institutions.

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Real-time error reporting systems allowing students to flag mistakes that publishers could fix in digital editions within 24 hours
Mandatory inclusion of QR codes linking to video tutorials for every concept, first pioneered by Pearson Education
The 'Math Marketplace' where teachers could vote on which theorems and problems should be included in future editions
Subscription-based access to customizable problem sets that adjusted difficulty based on individual student performance