[单选题]
Assuming the lower arm has a mass of 2.8 kg and it al 8.e9i-lytcs CG i ,-bh:3(dca4 c xahjcps 12 cm from the elbow-joint pivot, how much forca,a3-cc c(h4jbpdx :he must the extensor muscle in the upper arm exert on the lower arm to hold a 7.5 kg shot put (Fig. 9-1)?
