Which retail innovation was first widely implemented by the U.S. military during World War II that revolutionized how troops accessed products in combat zones?

Throughout military history, the Post Exchange (PX) and ship's stores have been vital for troop morale and readiness. These retail innovations have evolved dramatically from simple sutler wagons following Civil War regiments to today's modern military department stores. This poll tests your knowledge about a significant retail innovation that transformed how military personnel accessed consumer goods during a critical period in warfare history.

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Mobile Exchange Service (MEX) trucks that followed front lines
K-ration convenience packaging with integrated retail products
The 'Ship's Service' floating department stores on supply vessels
PX Ration Cards with points-based purchasing system for scarce goods