The correct option is C CO
Carbon-dioxide, carbon monoxide, oxides of sulphur and nitrogen are air pollutants which affect our health and environment badly.
Carbon dioxide causes global warming. Oxides of nitrogen and Sulphur causes acid rain.
Carbon-monoxide is a major air pollutant. It combines with hemoglobin in the human body and forms carboxyhemoglobin which reduces the oxygen absorption capacity of blood.