The electric motor exerts a $500-\mathrm{N} \cdot \mathrm{m}$ torque on the aluminum shaft $A B C D$ when it is rotating at a constant speed. Knowing that $G=27 \mathrm{GPa}$ and that the torques exerted on pulleys $B$ and $C$ are as shown, determine the angle of twist between $(a) B$ and $C$ (b) $B$ and $D$.