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(6) Two thin-walled steel structures having a same length with the cross-sections shown in the figure below are subjected to a same torque 7. Which of the following statements is incorrect for these thin-walled structures? 2R o (A) Structure (b) has smaller shear stress than structure (a). (B) The shear stress of structure (a) is 4 times the shear stress of structure (b). (C) Structure (b) has smaller deformation than structure (a). (D) The twisting angle of structure (a) is 16 times the twisting angle of structure (6). (7) An aluminum alloy block is fixed to two rigid plates at its top and bottom surfaces, respectively, as shown in the figure below. If the top plate displaces 1 mm horizontally when it is subjected to a horizontal force P = 500 kN while the bottom plate is stationary. The width of the block is 10 mm. Using the energy method, which of the following values is correct for the strain energy stored in the block material? 400 mm P= 500 KN 200 mm (A) 300 Nm (B) 300 Nm (C) 250 Nm (D) 150 Nm