A shaft of diameter 8cm is subjected to a bending moment of 3000 N m and a twisting moment of 4000 Nm. The maximum normal stress induced in the shaft is equal to
(a)$\frac{250}{\pi }MPa$ (b)$\frac{500}{\pi }MPa$
(c)$\frac{157.5}{\pi }MPa$ (d)$\frac{315}{\pi }MPa$