The rate and depth of normal breathing are determined by
group of answer choices

a. a pacemaker in the diaphragm.
b. a pacemaker in the intercostal muscles.
c. carbon dioxide levels in the blood.
d. oxygen levels in the blood.
e. carbon monoxide levels in the blood.

The rate and depth of normal breathing is determined by the amount of carbon dioxide in blood.

C. carbon dioxide levels in the blood.

Explanation:

Respiration process is the intake of oxygen and exhalation of carbon dioxide. Oxygen is the element required for cellular respiration. Carbon dioxide is the waste product and should be removed.  

The respiratory center in the brain is medulla and pons which control respiratory activities. Increased level of carbon dioxide in blood is unhealthy. In such a situation the respiratory center is simulated and the breathing rate and depth is increased.  

thinking it would be physical characteristics