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 ambient carbon dioxide levels on yawning. If yawning has a regulatory mechanism similar to breathing, then which of the following is TRUE?

a. People adapted to lower altitudes should yawn the same amount when they are at high altitudes.
b. Athletes who have trained at high altitudes should inhale a greater volume of air when yawning compared to those who have trained only at low altitudes.
c. Tibetan mountaineers yawn more when they are walking compared to when they are standing still.
d. Athletes should yawn more when exercising compared to when they are not exercising.