The $60-$mm$-$diameter A$-36$ steel shaft $(G=28$GPa $)$ is subjected to the torques shown. Determine $($a$)$ the maximum shear stress in segment $AB,($b$)$ the shear stress in member $BC$ at a point midway between its axis and surface, $($c$)$ the angle of twist of end A relative to $B,$ and $($d$)$ the angle of rotation of end $A.$