In a human body immune system, ______ secrete antibodies while ________ carry out cell-mediated responses. 

• A. Cytotoxic T cells; Effector B cells
• B. Helper T cells; Cytotoxic T cells
• C. Cytotoxic B cell; Cytotoxic T cells
• D. Effector B cells; Cytotoxic T cells

Answer: (D)