$(33$ points$)$ A torque of $1\mathrm{.}4kN-m$ is applied at point B $.$ The beam is made of steel $(G=77\mathrm{.}2$GPa $)$ and has a diameters of $50$ mm and $38$ mm as shown in the figure. Calculate:

a$)$ The internal torque in section $AB($state positive or negative$)$

b$)$ The internal torque in section $BC($state positive or negative$)$

c$)$ The angle of twist of point B $($relative to either end$)$