Light from a hydrogen discharge tube is incident on the cathode of a photoelectric cell the work function of the cathode surface is 4.2 eV. In order to reduce the photo-current to zero the voltage of the anode relative to the cathode must be made [DCE 2002]

• A.            ? 4.2 V                                    
• B.            ? 9.4 V
• C.            ? 17.8 V                                  
• D.            +9.4 V