Which mathematical algorithm forms the foundation of Amazon's original product recommendation system that revolutionized e-commerce personalization?

Online shopping has been transformed by recommendation systems that suggest products based on your browsing history, purchases, and similar customers' behaviors. Behind these personalized suggestions are sophisticated mathematical algorithms that analyze vast amounts of data. This poll tests your knowledge about the mathematical foundations that power the personalization features we encounter daily while shopping online.

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K-means clustering
Item-to-item collaborative filtering
Gradient boosting decision trees
Principal component analysis (PCA)