Given that $\cos \left( {\frac{{\alpha - \beta }}{2}} \right) = 2\cos \left( {\frac{{\alpha + B}}{2}} \right)$, then $\tan \frac{\alpha }{2}\tan \frac{\beta }{2} $ is equal to

  • A

    $\frac{1}{2}$

  • B

    $1\over3$

  • C

    $\frac{1}{4}$

  • D

    $\frac{1}{8}$

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