1. (1999 Henan Province Competition Team) 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
1. Choose C. Reason: Original expression $=\frac{\sqrt{\cos ^{2} 20^{\circ}+\sin ^{2} 20^{\circ}-2 \sin 20^{\circ} \cos 20^{\circ}}}{\cos 20^{\circ}-\sqrt{1-\cos ^{2} 20^{\circ}}}=\frac{\cos 20^{\circ}-\sin 20^{\circ}}{\cos 20^{\circ}-\sin 20^{\circ}}=1$.