Carbon Cycle 

A. the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of Earth.

B. a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into ATP

C. a non-living part of an ecosystem that shapes its environment

D. a proposed explanation made on the basis of limited evidence as a starting point for further investigation.