If $\theta = 3\, \alpha$ and $sin\, \theta =$ $\frac{a}{{\sqrt {{a^2}\,\, + \,\,{b^2}} }}$. The value of the expression , $a \,cosec\, \alpha - b \,sec\, \alpha$ is

  • A

    $\frac{1}{{\sqrt {{a^2}\,\, + \,\,{b^2}} }}$

  • B

    $2 \sqrt {{a^2}\,\, + \,\,{b^2}}$

  • C

    $a + b$

  • D

    none

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