The answer is d. (Hardman, pp 1676–1678. Katzung, pp 990–991.)
Carbon monoxide is a common cause of accidental and suicidal poisoning. Its affinity for hemoglobin is 250 times greater than that of O2. It therefore binds to hemoglobin and reduces the O2-carrying capacity of blood. The symptoms of poisoning are due to tissue hypoxia; they progress from headache and fatigue to confusion, syncope, tachycardia, coma, convulsions, shock, respiratory depression, and cardiovascular collapse. Carboxyhemoglobin levels below 15% rarely produce symptoms; above 40%, symptoms become severe. Treatment includes establishment of an airway, supportive therapy, and administration of 100% O2. Sulfur dioxide, ozone, and nitrogen dioxide are mucous membrane and respiratory irritants. Methane is a simple asphyxiant.