Exposure to carbon monoxide (from coal gas) is extremely dangerous and can kill a patient because

• A
  The compound carboxyhaemoglobin $(COHb)$ it forms with haemoglobin can gradually clot the blood resulting in circulatory failure
• B
  $COHb$ reduces the ability of blood to transport oxygen by rupturing a vast majority of erythrocytes
• C
  $COHb$ greatly modifies the structure of haemoglobin,thus making it lose its affinity for oxygen
• D
  None of the above