SCID is defined by a lack of functional B and T cells which eliminates protection by humoral and cell-mediated immunity. Which of the following is true?

A. A genetic defect disrupts the development of the lymphocytic cell lines.

B. A genetic mutation renders the individual incapable of removing a toxic DNA metabolite from lymphocytes, which ultimately destroys them

C. Genes coding for B and T cell receptors are mutated so the cells are incapable of binding antigens.

D. All of the above describe cases of SCID.