Which innovative packaging feature was specifically introduced in the U.S. Army's K-Ration during World War II that revolutionized field feeding?

During World War II, military logistics and product packaging underwent revolutionary changes to meet the demands of global warfare. One of the most significant innovations was the development of combat rations that could withstand extreme conditions while providing adequate nutrition to soldiers in the field. This trivia question explores how product packaging design played a crucial role in military history and potentially affected battle outcomes.

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Wax-coated cardboard boxes that could be used as emergency fuel for heating
Self-heating containers using chemical reactions to warm food
Biodegradable packaging that could be buried to avoid detection
Pressurized cans that preserved food without refrigeration for up to 5 years