Which of the following statements is not correct about a cloning vector?

• A
  $Ori$ is a sequence responsible for controlling the copy number of the linked $DNA$.
• B
  Selectable marker selectively permitting the growth of the non-transformants.
• C
  In order to link the alien $DNA$,the vector needs to have a single recognition site for the commonly used restriction enzymes.
• D
  The ligation of alien $DNA$ is carried out at a restriction site present in one of the two antibiotic resistance genes.