The boring bar of a boring machine is 25 mm in diameter. During operation, the bar gets twisted though 0.01 radians and is subjected to a shear stress of 42 N / mm2 . The length of the bar is (Taking $G = 0.84\times 10^{5}N/mm^{2}$)
(a) 500 mm (b) 250 mm
(c) 625 mm (d) 375 mm