5. $\tan 70^{\circ} \cdot \cos 10^{\circ}\left(\sqrt{3} \tan 20^{\circ}-1\right)=$ ).
(A) 1
(B) 2
(C) -1
(D) -2
$\begin{array}{l}\text { 5. C. } \\ \text { Original expression }=\frac{\cos 20^{\circ} \cdot \cos 10^{\circ}}{\sin 20^{\circ}} \cdot \frac{\sqrt{3} \sin 20^{\circ}-\cos 20^{\circ}}{\cos 20^{\circ}} \\ =\frac{\cos 10^{\circ} \cdot 2 \sin \left(20^{\circ}-30^{\circ}\right)}{\sin 20^{\circ}} \\ =-\frac{2 \cos 10^{\circ} \cdot \sin 10^{\circ}}{\sin 20^{\circ}}=-1 .\end{array}$