The set of all \( \alpha \), for which the vectors \( \vec{a} = \alpha t \hat{i} + 6\hat{j} - 3\hat{k} \) and \( \vec{b} = t\hat{i} - 2\hat{j} - 2\alpha t \hat{k} \) are inclined an obtuse angle for all \( t \in \mathbb{R} \), is:
\( \left(-\frac{4}{3},\, 1\right] \)
\( [0,\, 1) \)
\( (-2,\, 0] \)
\( \left(-\frac{4}{3},\, 0\right] \)