Multiple choice

A spherical balloon of radius a units subtends an angle $\theta$, at an observer's eye. The angle of elevation of centre of the balloon is $\phi$. The height of the centre of the ball is:

$a \sin \theta \,cosec \dfrac{\phi}{2}$
$a \,\sin \dfrac{\phi}{2} \,cosec \phi$
$a \,\sin \phi \,cosec\dfrac{\phi}{2}$
$a \,\sin \dfrac{\phi}{2} \,cosec \theta$

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