Answer:
In animals, oxygen combines with food in the cells to produce energy for daily activity and then gives off carbon. The carbon combines with oxygen to form carbon dioxide (CO2) and is released back into the atmosphere as a waste product when animals breathe and exhale.
The actual answer is, "A", an ocean trench.
You can find out more about them by researching them up on Google, it's only a few phrases and a hit on the enter key :P
