Correct Answer - C
`CO` is very poisonous because it has a `300 `- fold greater affinity for blood hemoglobin than does oxygen:
Hemoglobin + `CO rarr` Carboxyhemoglobin
`"Hemoglobin"+O_(2)underset(larr)(rarr)"Oxyhemoglobin"`
Thus, quite low concentrations of `CO` in air are sufficient to prevent oxygen absorption in the lungs. Without a continuous supply of oxygen, the brain loses consciousness and death follows (due to asphyxiation) unless the supply of oxygenated hemoglobin is restored.