Answer:
a. Carbon
Step-by-step explanation:
During volcanic outgassing is a process whereby volcanic eruptions emits gases to the atmosphere. Examples of gases released are carbon dioxide and water vapor. The respiration of animal takes in oxygen and gives out carbon dioxide. Flatulence is the release of gas (i.e. carbon dioxide) from the digestive system out of the back passage. The burning of fossil fuel also releases carbon dioxide. Metamorphism of limestone is when limestone is subjected to pressure and heat. Carbon is the element that is present in all these processes.