If $\sin A = n\sin B,$ then $\frac{{n - 1}}{{n + 1}}\tan \,\frac{{A + B}}{2} = $

  • A

    $\sin \frac{{A - B}}{2}$

  • B

    $\tan \frac{{A - B}}{2}$

  • C

    $\cot \frac{{A - B}}{2}$

  • D

    None of these

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