Assertion $(S)$: Viruses are considered as a link between living and non-living organisms.
Reason $(R)$: Viruses replicate only when they are inside a host cell,whereas they behave as non-living particles in a free state.

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