CO is harmful because
A. It forms a stable compound with hemoglobin
B. It blocks mitosis
C. It is mutagenic
D. It causes defoliation

Hint: CO stands for Carbon monoxide. Carbon monoxide is a harmful poisonous gas for humans. Carbon monoxide is found in the smoke that is released by the burning of wood, fossil fuels, or the fumes released by the exhaust of the vehicles.

Complete answer: Carbon monoxide is harmful to human beings. This is because the haemoglobin found in the human blood has a very great affinity for carbon monoxide as compared to oxygen. The hemoglobin has two hundred times more affinity for carbon monoxide as compared to oxygen. The haemoglobin binds with carbon monoxide to form a stable compound known as carboxyhaemoglobin. When there is excessive carbon monoxide in the surroundings and a person inhales it then this compound leads to the deficiency of oxygen in the body. Due to this reason, carbon monoxide poisoning occurs and the person dies. Therefore, carbon monoxide is harmful to humans.
Thus, based on the above information we can conclude that CO is harmful because It forms a stable compound with hemoglobin.
Hence, the correct answer is option (A).

Note: Carboxyhemoglobin should not be confused with carbamino hemoglobin. Carbamino hemoglobin is the compound that is formed by the reaction of the carbon dioxide with hemoglobin of the red blood cells. Carbon dioxide, however, has a lesser affinity for hemoglobin as compared to oxygen. Its diffusibility is twenty times more in blood as compared to oxygen. This carbamino hemoglobin is formed in the body during the various respiration processes. It is not harmful to the body and it is released as a by-product.