Correct Option: A

Tobacco smoke contains carbon monoxide which is a poisonous gas. It interferes with uptake of oxygen in the lungs and with its release from the blood to the tissues that need it. When carbon monoxide is inhaled it combines with haemoglobin in the blood to form carboxyhaemoglobin, which reduces the amount of oxygen available to the body's vital organs. Oxygen levels may be reduced by as much as 15%.