Mark the tick against the correct answer in the following: \left|\begin{array}{ll} \cos 15^{\circ} & \sin 15^{\circ} \\ \sin 15^{\circ} & \cos 15^{\circ} \end{array}\right|=?
A. 1
B. \frac{1}{2}
C. \frac{\sqrt{3}}{2}
D. none of these