The maximum circumferential stress in the cylinder will occur at a radius of

Question

A thick – walled hollow cylinder having outside and inside radii of 90 mm and 40 mm respectively is subjected to an external pressure of 800$MN/m^{2}$ . The maximum circumferential stress in the cylinder will occur at a radius of                                                  

         (a) 40 mm               (b) 60 mm

         (c) 65 mm               (d) 90 mm