The set of all values of \( k \) for which \( (\tan^{-1}x)^{3} + (\cot^{-1}x)^{3} = k\pi^{3},\ x \in \mathbb{R} \), is the interval:
\( \left[ \frac{1}{32}, \frac{7}{8} \right) \)
\( \left( \frac{1}{24}, \frac{13}{16} \right) \)
\( \left[ \frac{1}{48}, \frac{13}{16} \right] \)
\( \left[ \frac{1}{32}, \frac{9}{8} \right) \)