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

Throughout history, retail strategies have evolved dramatically to meet changing consumer demands. From the ancient marketplaces to modern e-commerce, certain innovations have transformed how we shop. This trivia question focuses on a pivotal development in retail strategy that revolutionized the consumer experience in the 20th century and continues to influence modern retail approaches.

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Self-service shopping (the modern supermarket model)
Mail-order catalogs with home delivery
Department stores with multiple specialized sections
Installment payment plans for major purchases