I would say oxygen because respiration is breathing mechanically.
I think it’s C an increase in producer population
Answer:
Cellular Respiration: a process able to extract a large amount of energy from food molecules. In eukaryotic cells oxygen is a required component. This metabolic process is the main reason that animals have elaborate gas exchange organs such as lungs, gills and other systems. The goal of these systems is, of course, to get access to oxygen and to get rid of carbon dioxide.
Explanation:
