In gene therapy of Adenosine Deaminase $(ADA)$ deficiency,the patient requires periodic infusion of genetically engineered lymphocytes because

• A
  Gene isolated from marrow cells producing $ADA$ is introduced into cells at embryonic stages.
• B
  Lymphocytes from patient's blood are grown in culture,outside the body.
• C
  Genetically engineered lymphocytes are not immortal cells.
• D
  Retroviral vector is introduced into these lymphocytes.