A circular solid shaft is subjected to bending moment of 400kN – m and a twisting moment of 300kN – m. one the basis of the maximum principal stress theory, the direct stress is 
and according to the maximum shear stress theory, the shear stress is $\tau$ . The ratio $\frac{\sigma }{\tau }$ is
(a) 1/5 (b) 3/9
(c) 9/5 (d) 11/6