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

The retail landscape has transformed dramatically throughout history, with technological innovations and changing consumer behaviors reshaping how merchants sell their goods. From ancient marketplaces to modern e-commerce giants, retail strategies have evolved in response to societal changes. This trivia question explores a significant historical innovation that fundamentally changed retail operations and consumer shopping habits in the 20th century.

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Self-service grocery stores (pioneered by Piggly Wiggly)
Department store catalogs with mail ordering
Coin-operated vending machines
Membership-based warehouse clubs