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