A close-coiled helical spring of stiffness 30 N/mm is arranged in series with another such spring of stiffness 60 N/mm. The stiffness of composite unit is
(a) 20 N/mm
(b) 30 N/mm
(c) 45 N/mm
(d) 90 N/mm
(a) 20 N/mm
(b) 30 N/mm
(c) 45 N/mm
(d) 90 N/mm