In a $\Delta ABC$, if AB = AC and BC is extended to D such that $\angle{A}CD = 100^o$, then $\angle{A}$ is

Question

$\begin{array}{1 1} (a)\;20^{\circ} \\ (b)\; 40^{\circ} \\ (c)\; 60^{\circ} \\ (d)\; 80^{\circ} \end{array} $