Let \( S = \left\{ \theta \in \left(0, \dfrac{\pi}{2}\right) : \displaystyle\sum_{m=1}^{9} \sec\left(\theta + (m-1)\frac{\pi}{6}\right)\sec\left(\theta + \frac{m\pi}{6}\right) = -\frac{8}{\sqrt{3}} \right\} \). Then
\( S = \left\{ \dfrac{\pi}{12} \right\} \)
\( S = \left\{ \dfrac{2\pi}{3} \right\} \)
\( \displaystyle\sum_{\theta \in S} \theta = \frac{\pi}{2} \)
\(\displaystyle \sum_{\theta \in S} \theta = \frac{3\pi}{4} \)