$CO$ (Carbon monoxide) is a neutral oxide of carbon,but it acts as a very strong field ligand and associates with metals to form complex compounds. These complex compounds are very useful in metallurgical processes. $CO$ is a poisonous gas because:

• A
  It forms a complex with hemoglobin,which is incapable of carrying oxygen.
• B
  It affects the lungs.
• C
  It decomposes the protein part of our body.
• D
  It associates with $H_2$ to form a gaseous mixture which is very lethal.