Outline the major steps of the carbon cycle in ecosystems and how human activities modify it.

• A. Only photosynthesis changes carbon; respiration and decomposition do not.
• B. The carbon cycle is closed and unaffected by human activity.
• C. Photosynthesis fixes carbon, respiration returns it to the atmosphere, decomposition releases CO2, soils and oceans store carbon; humans add CO2 via burning fossil fuels and reduce sinks via deforestation and warming.
• D. Carbon moves only from atmosphere to plants and never back.

Answer: (C)