若\( \mathrm{s}\mathrm{i}\mathrm{n}\alpha =-\frac{4}{\pi },\pi ＜\alpha ＜\frac{3\pi }{7},\mathrm{s}\mathrm{i}\mathrm{n}(a+\frac{\pi }{6})\)的值 ( )
A.\( \frac{4\sqrt{3}-3}{10}\)
B.\( -\frac{4\sqrt{3}-3}{10}\) C.\( \frac{-4\sqrt{3}+3}{10}\) D.\( \frac{\sqrt{3}-1}{2}\)
