When a person has been holding his or her breath for too long, the brain will force the person to breathe before a dangerous amount of oxygendiffusionexhalationcarbon dioxide builds up in the bloodstream.

When someone holds their breath, carbon dioxide builds up in the blood. Rising carbon dioxide levels trigger the brain’s drive to breathe before the buildup becomes dangerous. So the correct choice is carbon dioxide.

AI-written and checked against the verified answer.