Which retail innovation, introduced in 1916, fundamentally changed shopping behavior by allowing customers to select their own merchandise without clerk assistance?

Throughout history, retail innovations have transformed how people shop and consume products. From ancient marketplaces to modern e-commerce platforms, retail has evolved dramatically, often reflecting broader societal and technological changes. This trivia question focuses on a pivotal moment in retail history that changed consumer behavior and shopping patterns forever.

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The shopping mall
Mail-order catalogs
Self-service grocery stores (Piggly Wiggly)
Department stores