Choose the correct statement:
$P -$ Endonucleases cut the $DNA$ at a specific site inside the $DNA$.
$Q -$ $A$ stirred-tank reactor is usually square in shape.
$R -$ In order to force bacteria to take up the plasmid,the bacterial cells must first be made 'competent' to take up $DNA$ by treating them with a divalent cation like $Ca^{2+}$.

• A
  Statement $P$ is true
• B
  Statement $Q, R$ are true
• C
  Statement $P, Q$ are true
• D
  Statement $P, Q, R$ are true