Answer:
oxygen
Explanation:
The Carboniferous period is a period that stretched from 358.9 million years ago to 298.9 million years ago, being on the scene for 60 million years. This period is characterized mostly by its warm and wet climate, dense tropical rainforests, giant insects, myriapods, and arachnids roaming the land. The reason why in this period the plants grew very tall, some over 30 meters, and animals that are otherwise very small became significantly larger, is that during this period the amount of oxygen in the atmosphere was much higher. The high amount of oxygen in the atmosphere came from all the plants, as the majority of the land was covered with plants until the late stages of this period. The high amount of oxygen in the atmosphere though also killed big portion of the flora and terrestrial fauna at the end of this period because it cooled off the atmosphere significantly, thus the warm and wet climate was replaced by glaciation, cold, and dry climate.
When World War II was over it came to light that the military had rejected over half of the men who were drafted or had given them lighter duties, because they were not fit enough for combat. The United States began to realize it needed to focus more on fitness. A researcher by the name of Thomas K. Cureton began to study exercise and fitness. He conducted many experiments and was able to establish a set of guidelines for maintaining fitness.
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