$A$ - Cloning vectors are useful to transfer desirable $DNA$ into host cells.
$R$ - Agrobacterium tumefaciens is a pathogen of several dicot plants.

• A
  $A$ and $R$ both are correct.
• B
  $A$ and $R$ both are incorrect.
• C
  $A$ is correct, $R$ is incorrect.
• D
  $A$ is incorrect, $R$ is correct.