$A$: Viruses are considered as a link between living and non-living organisms.
$R$: Viruses behave as inert,non-living particles in a free state,while they exhibit active,living behavior inside a host cell.

• A
  $A$ and $R$ are both true,and $R$ is the correct explanation of $A$.
• B
  $A$ and $R$ are both 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.