For the given plane stress state in Question $6$ above, $\left[\begin{array}{ccc}90& 55& 0\\ 55& -50& 0\\ 0& 0& 0\end{array}\right]$MPa, the

octahedral normal stress is

a$.$

$13\mathrm{.}3$ MPa

b$.$

$20$MPa

c$.$

$73\mathrm{.}3$ MPa

d$.$

$40$MPa

e$.$

$90$MPa