[单选题]
Assuming the lower arm has a mass of 2.8 kg and its CG is am-n0gywh ; a7 i4-o5fqqli k,gpr v4de 2j44 eu2oaz,12 cm from the elbow-joint pivot, how much force must the extensor muscle in the upper arm exert on the lower arm to hold a 7.5 kg shot put2p2gzo , ua44rd 4ejev (Fig. 9-1)?
