The equivalent spring stiffness of the two springs is nearly

Question

A mass is suspended at the bottom of two springs in series having stiffness 10 N/mm and 5 N/mm. The equivalent spring stiffness of the two springs is nearly                                                             

         (a) 0.3 N/mm                  (b) 3.3 N/mm             

         (c) 5 N/mm                     (d) 15 N/mm