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. 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.

The correct answer in the blank is arterial blood.

The elevated levels of CO2 in the arterial blood signals the brain to increase the rate and depth of inhalations and exhalations.

Blood in the veins normally has the high concentration of CO2.

Arterial blood contains very low level of CO2 and High level of CO2 in this blood signals brain to increase breathing rate.

chordata is the answer

explanation:

the universal gravitation equation. since the gravitational force is directly proportional to the mass of both interacting objects, more massive objects will attract each other with a greater gravitational force. if the mass of one of the objects is doubled, then the force of gravity between them is doubled.