Which part of the carbon cycle occurs when plants, trees, or fossil fuels are burned? A. Transpiration B. Oxidation C. Photosynthesis D. Combustion

Combustion.

Explanation:

It's just as it sounds, combustion is the process of burning something. In the carbon cycle, combustion occurs whenever any organic living or non-living thing is burned in the proximity of oxygen, afterwards producing carbon dioxide.

the answer is a or when it says crossing over

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."