\textbf{139.} If $\begin{bmatrix} \cos 15° & \sin 15° \\ -\sin 15° & \cos 15° \end{bmatrix}^n = -I$, what is the smallest natural number $n$? \medskip (1) $6$ \hfill (2) $12$ \hfill (3) $18$ \hfill (4) $24$ \bigskip