Lines $x$ and $y$ are parallel to each other as shown in the figure.What can be the value of $b-a$ ?

Question

$\begin{array}{1 1} (a)\; 30^{\circ} \\ (b)\; 120^{\circ} \\ (c)\; 60^{\circ} \\ (d)\; 100^{\circ} \end{array}$