已知 $\cos (\alpha -\frac{\pi }{6})+\sin \alpha =\frac{4}{5}\sqrt{3}$，则 $\sin (\alpha +\frac{7\pi }{6})$的值是
A. $-\frac{2\sqrt{3}}{5}$  B. $\frac{2\sqrt{3}}{5}$ C. $-\frac{4}{5}$ D. $\frac{4}{5}$