$A.$ The capsid of a virus is made up of many small units called capsomeres.
$R.$ Viruses exhibit active and living behavior in a virus-free state.

• 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.