In the figure $\overline{AB} \parallel \overline{CD}$ and $\overline{AD}$ bisects $\angle CAB$, what is the measure of $\angle ADC$ ?

Question

$\begin{array}{1 1} (a)\;80^{\circ} \\ (b)\;100^{\circ} \\ (c)\;40^{\circ} \\(d)\;120^{\circ} \end{array} $