Which retail innovation, first widely implemented in the early 1900s, revolutionized shopping by allowing customers to select their own merchandise without clerk assistance?

Throughout history, retail innovations have fundamentally changed how people shop and consume. From ancient marketplaces to digital commerce, these transformations reflect broader societal and technological changes. This question focuses on one of the most significant retail format innovations that shaped modern consumer culture and revolutionized how products were sold in the early 20th century.

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Self-service grocery stores (pioneered by Piggly Wiggly in 1916)
Department stores with escalators (introduced by Harrods in 1898)
Mail-order catalogs (popularized by Montgomery Ward in the 1870s)
Automated vending machines (first commercial deployment in London in 1883)