Consider the following.
Assertion $(A)$: $CO$ is poisonous to living beings.
Reason $(R)$: $CO$ binds to haemoglobin forming carboxyhaemoglobin,which is less stable than oxygen-haemoglobin complex.
Correct answer is

• A
  $A$ and $R$ both are correct and $R$ is the correct explanation for $A$
• B
  $A$ and $R$ both are correct and $R$ is not the correct explanation for $A$
• C
  $A$ is correct,but $R$ is not correct
• D
  $A$ is not correct,but $R$ is correct