Scenario Yawning Is a Reflexive Action That Is Often Associated with with Sleepiness

Scenario
Yawning is a reflexive action that is often associated with sleepiness. When a person yawns, the mouth opens, and a longer than normal breath of air is inhaled, followed by a longer than normal exhalation. Hypothesizing that yawning has a regulatory mechanism similar to breathing, scientists conducted an experiment on college students to test the effect of existing carbon dioxide levels on yawning.
-The scientists investigated carbon dioxide levels in their experiment because high carbon dioxide levels in the ________ signal control centers in the brain to increase rate and depth of inhalations and exhalations.

A) alveoli
B) blood
C) heart muscle
D) pharynx