When a patient with defective ADA is treated, which of the following steps are performed for gene therapy?
(A) Lymphocytes are obtained from the patients.
(B) Lymphocytes are transferred to culture dishes.
(C) Lymphocytes are transected with normal ADA genes.
(D) The transected cells are returned to the patients.
(a) All the above
(b) Only $\mathrm{C}$ and $\mathrm{D}$
(c) Only D
(d) SCID cannot be treated