If the value of the integral \( \displaystyle \int_{-1}^{1} \frac{\cos\alpha x}{1 + 3^{x}} \, dx \) is \( \dfrac{2}{\pi} \). Then, a value of \( \alpha \) is:
\( \dfrac{\pi}{4} \)
\( \dfrac{\pi}{2} \)
\( \dfrac{\pi}{3} \)
\( \dfrac{\pi}{6} \)