Which retail innovation, first widely implemented in the 1910s and 1920s, transformed shopping by allowing customers to select their own merchandise without clerk assistance?

Throughout history, retail has undergone several transformative revolutions that changed how consumers shop. From ancient marketplaces to modern digital commerce, retail innovations have shaped economies and societies. This trivia question focuses on a pivotal moment in retail history that fundamentally altered the customer experience and set the stage for modern subscription and self-service models.

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Mail-order catalogs
Self-service grocery stores (Piggly Wiggly model)
Department stores with multiple sections
Subscription-based purchasing clubs