The Carbon Cycle can be those two :
The animals breath out carbon dioxide into the atmosphere. Then the plants take the carbon dioxide because they use it as energy. After the plants take the carbon dioxide, they produce oxygen as photosynthesis. The animals breath in oxygen and breath out carbon dioxide. The natural cycle continued.
When an animal or a plant die, it decomposed into the ground. After a long time, the animals and plants goes down and down into the deeper layers. The heat under the ground makes the animals and plants melt as coal, natural gas and oil. Those things are called fossil fuel. Then the human started digging fossil fuels as human activities. The carbon dioxide was been produced by human activities so they rise up to the atmosphere. Then the plants take the carbon dioxide and produced the oxygen. The human cycle continued.