[单选题]
Assuming the lower arm has a mass ofg rs2+s*m f7cw j-4jh5(j* anx.jsi jb 2.8 kg and its CG is 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 bahj5-4 jxjj .sjin(*.5 kg shot put (Fig. 9-1)?
