If $a \tan \alpha + b \tan \beta = (a + b) \tan \left( \frac{\alpha + \beta}{2} \right)$ and $\alpha - \beta \neq 2n\pi$,then $\frac{\cos \beta}{\cos \alpha} = $

  • A
    $\frac{a}{b}$
  • B
    $\frac{a + b}{a - b}$
  • C
    $\frac{a^2 - b^2}{a^2 + b^2}$
  • D
    $\frac{b}{a}$

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