In the figure, at the free end of the light string, a force F is applied to keep the suspended mass of 18 kg at rest. Then the force exerted by the ceiling on the system (assume that the string segments are vertical and the pulleys are light and smooth) is: (g= 10 m/s2)
(A) 60 N
(B) 120 N
(C) 180 N
(D) 240 N
