[单选题]
Assuming the lower arm has a mass of 2.8 kg and its CG is 12 cm from the e cyfxb*uom8 3cpi* r(6lbon7bxgk/) *b6-z rhpfxw-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 put (Fig. 9-1 rx) fzg-6*bk 7nbhxp/)?
