Which retail innovation, introduced in the 1930s, revolutionized shopping by allowing customers to select their own merchandise without clerk assistance for the first time?

The retail landscape has transformed dramatically throughout history. Before department stores, shopping malls, and e-commerce, retail innovations shaped how people purchased goods and revolutionized consumer culture. This trivia question explores a pivotal moment in retail history that changed shopping forever and set the foundation for modern commerce practices we take for granted today.

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Self-service shopping carts (introduced by Sylvan Goldman in Oklahoma)
Self-service supermarkets (pioneered by Piggly Wiggly in 1916)
Cash registers with receipt printing (developed by NCR in the 1920s)
Department store credit cards (first offered by Gimbels in 1938)