Let $\theta$ be the angle between two unit vectors $\hat{a}$ and $\hat{b}$ such that $\sin \theta = \frac{3}{5}$. Then, $\hat{a} \cdot \hat{b}$ is equal to : (A) $\pm \frac{3}{5}$ (B) $\pm \frac{3}{4}$ (C) $\pm \frac{4}{5}$ (D) $\pm \frac{4}{3}$