若 $\tan \alpha=3$, 则 $2 \sin \alpha \cos \alpha=(\quad)$
$2 \sin \alpha \cos \alpha=\frac{2 \sin \alpha \cos \alpha}{\sin ^{2} \alpha+\cos ^{2} \alpha}=\frac{2 \tan \alpha}{\tan ^{2} \alpha+1}=\frac{6}{10}=\frac{3}{5}$.