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

Throughout history, retail strategies have evolved dramatically, transforming how people shop and businesses operate. From ancient marketplaces to today's digital commerce, innovative retail concepts have shaped economies and societies. This trivia question explores a pivotal moment in retail history that fundamentally changed consumer shopping habits and launched a new era in merchandising that we still experience today.

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Self-service shopping carts (introduced by Sylvan Goldman at Humpty Dumpty Supermarket)
Self-service grocery stores (pioneered by Piggly Wiggly's Clarence Saunders)
Department store catalogs (popularized by Montgomery Ward)
Retail loyalty programs (started by S&H Green Stamps)