Assertion $(A)$: The pathogen responsible for Parkinson's disease in humans is viroids.
Reason $(R)$: Viroids are smaller in size than viruses.

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