What is the primary physiological effect of carbon monoxide exposure on blood?

• A. It reduces the oxygen-carrying capacity of hemoglobin by forming carboxyhemoglobin
• B. It increases dopamine release in the brain to create pleasurable sensations 
• C. It lowers circulating thyroid hormone levels by damaging the thyroid gland 
• D. It primarily causes irritation and inflammation of the upper airways

Answer: (A)