Which retail innovation, first implemented in 1916 by Clarence Saunders, revolutionized shopping by allowing customers to select their own merchandise without clerk assistance?

Throughout history, certain product innovations have fundamentally transformed how retail operates and how consumers shop. These breakthrough products didn't just succeed in the market—they revolutionized entire industries and shopping behaviors. This trivia question focuses on one such pivotal innovation from the early 20th century that laid the groundwork for modern self-service retail as we know it today.

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Piggly Wiggly (the first true self-service grocery store)
The shopping cart (invented by Sylvan Goldman)
The barcode scanner (patented by Norman Woodland)
The cash register (invented by James Ritty)