The act of breathing raises the blood oxygen levels, lowers the blood carbon dioxide concentration, and raises the blood pH. According to the principles of negative feedback, sensors that regulate the breathing should respond to A. a rise in blood oxygen
B. a rise in blood pH
C. a rise in blood carbon dioxide concentration
D. all of these

C. a rise in blood carbon dioxide concentration

Explanation:

Negative feed back mechanism refers to the control mechanism in which a product of a particular pathway controls the switch on and switch off of that particular pathway.

Since breathing increases the blood oxygen concentration and lowers carbon dioxide concentration and pH. The sensors that regulate the breathing should respond to oxygen concentration as the carbon dioxide is the by-product of respiration and is responsible for increasing blood pH. Thus  increased Carbon dioxide levels in blood is the only factor which will trigger breathing.

i believe it would be the lungs.

not sure if this what   you'r looking for would be percent composition