$A.$ Viroids are the pathogens responsible for Parkinson's disease in humans.
$R.$ Viroids are smaller in size than viruses.

• A
  Both $A$ and $R$ are true,and $R$ is the correct explanation of $A$.
• B
  Both $A$ and $R$ are true,but $R$ is not the correct explanation of $A$.
• C
  $A$ is true and $R$ is false.
• D
  $A$ is false and $R$ is true.