Let \( S \) be the set of positive integral values of \( a \) for which \( \dfrac{a x^{2}+2(a+1)x+9a+4}{x^{2}-8x+32}<0 \, \forall x \in \mathbb{R} \). Then, the number of elements in \( S \) is:
\( 3 \)
\( 0 \)
\( 1 \)
\( \infty \)