Which retail innovation, first implemented in 1916 at the Piggly Wiggly store in Memphis, Tennessee, fundamentally changed how customers shop for food?

The food retail industry has undergone numerous transformations over the decades, from simple corner stores to massive supermarkets. Point of sale systems, self-checkout, and other innovations have changed how we shop for food. This trivia question focuses on a pivotal innovation that forever changed how consumers interact with grocery products and revolutionized the entire retail food industry.

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Self-service shopping (allowing customers to select their own items rather than requesting them from clerks)
Introduction of shopping carts with wheels
The first barcode scanner system for inventory tracking
Price tags printed directly on products instead of verbal pricing