1. Simplify $\frac{\sqrt{1-2 \sin 20^{\circ} \cos 20^{\circ}}}{\cos 20^{\circ}-\sqrt{1-\cos ^{2} 160^{\circ}}}$ to get ( ).
(A) $\sqrt{1-\sin 40^{\circ}}$
(B) $\frac{1}{\cos 20^{\circ}-\sin 20^{\circ}}$
(C) 1
(D) -1
\begin{array}{l}\text {-.1.C. } \\ \begin{aligned} \text { Original expression } & =\frac{\sqrt{\cos ^{2} 20^{\circ}+\sin ^{2} 20^{\circ}-2 \sin 20^{\circ} \cos 20^{\circ}}}{\cos 20^{\circ}-\sqrt{1-\cos ^{2} 20^{\circ}}} \\ & =\frac{\cos 20^{\circ}-\sin 20^{\circ}}{\cos 20^{\circ}-\sin 20^{\circ}}=1 .\end{aligned}\end{array}