In the fixedly combined circular shaft shown in the figure, the central part of the material is steel, shear modulus G1=80GPa, the outer material is copper and the shear modulus G2=39GPa. The distribution of shearing stress on cross section is___after torsional deformation. ① ② ③ ④http://image.zhihuishu.com/zhs/onlineexam/ueditor/201904/e48a2be70d9f4eec8d38b0270076b6e2.png