In a Hartnell governor, the mass of each ball is 4 kg. The maximum and minimum centrifugal forces on the basis are 1800 N and 100 N at radii 25 cm and 20 cm, respectively. The lengths of vertical and horizontal arms of the bell-crank levers are the same. What is the spring stiffness?
(a) 780 N/cm (b) 740 N/cm
(c) 720 N/cm (d) 680 N/cm