Which innovative product strategy revolutionized college mathematics textbook sales in the early 2000s?

The mathematics textbook industry has evolved dramatically with changing educational needs and technological advances. Publishers have had to adapt their product development and marketing strategies to meet the demands of digital learning while maintaining educational rigor. This trivia question explores a significant turning point in how mathematics educational products were developed and marketed to students and institutions.

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Bundling textbooks with personalized adaptive learning software that adjusted problem difficulty based on student performance
Creating limited-edition designer covers for calculus textbooks endorsed by famous mathematicians
Implementing a subscription model where students could only access content for one semester at a time
Developing mathematics textbooks with augmented reality features that required special glasses to view 3D models