Which military retail innovation was first introduced during World War II and revolutionized how troops accessed products in combat zones?

Post Exchange (PX) stores have been a crucial part of military life for generations, providing essential goods and comfort items to service members worldwide. These retail operations have evolved dramatically from simple sutler's wagons following troops in the 1800s to today's modern department store equivalents. Test your knowledge about this unique retail system that has supported military personnel and their families through wars and peacetime.

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K-Rations with built-in currency for exchange purchases
Mobile PX trucks that brought retail goods directly to frontline troops
Catalog ordering system with delivery to field post offices
Punch card system for tracking soldier purchases and allowances