Answer:
D. increase of carbon dioxide
Step-by-step explanation:
Breathing is automatic and is controlled subconsciously by respiratory center at base of brain. It continues during the time of the sleep and even when the person is not in conscious. Sensory organs in brain and in aorta and the carotid arteries monitor blood and sense the oxygen and the carbon dioxide levels. Usually, increased concentration of the carbon dioxide is the strongest stimulus for the breathing to occur more deeply and more frequently.
When carbon dioxide concentration in the blood is high, the brain increases the frequency and the depth of breaths.