计算下列各式, 结果为 $\sqrt{3}$ 的是
$\text{A.}$ $\sqrt{2} \sin 15^{\circ}+\sqrt{2} \cos 15^{\circ}$
$\text{B.}$ $\cos ^2 15^{\circ}-\sin 15^{\circ} \cos 75^{\circ}$
$\text{C.}$ $\frac{\tan 15^{\circ}}{1-\tan ^2 15^{\circ}}$
$\text{D.}$ $\frac{1+\tan 15^{\circ}}{1-\tan 15^{\circ}}$