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19.2 Starting from rest, an object of mass m = 4kg slides down a hill of height h = 7m. The only forces that act on the object are gravity, friction, and the normal force. If the speed at the bottom of the hill is given by v=9m/s, what is the work done by the friction force is Wf? (A) 138 ] (B) -274 J (C) -63 J (D) -75 J (E) -112 J