Which pioneering recommendation algorithm first revolutionized online book retail in the late 1990s, creating the 'customers who bought this also bought' feature?

Before recommendation algorithms revolutionized online bookstores, physical bookshops relied on various techniques to connect readers with their next favorite book. This poll explores the fascinating history of book recommendation systems and how they've evolved in the retail literature space. Test your knowledge about the pioneering algorithm that changed how we discover books forever!

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Item-based collaborative filtering (developed by Amazon)
PageRank (developed by Google)
Content-based filtering (developed by Barnes & Noble)
Neural network recommendations (developed by Borders)