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 found that the level of carbon dioxide in the air did not cause college students to yawn more frequently. What other experiment could they conduct to confirm that they should reject the hypothesis that yawning is regulated in the same way as breathing?

A) Test the effect of hemoglobin amounts on the frequency of yawning.
B) Test the effect of exercise levels on the frequency of yawning.
C) Test the effect of carbon monoxide (CO) levels on the frequency of yawning.