Shots are fired simultaneously from the top and bottom of a vertical cliff at angles $\alpha$, and $\beta$ and they strike an object simultaneously at the same point. Show that if the horizontal distance of the object from the cliff is $a$, the height of the cliff is
(a) $\frac{a(\cot \alpha-\cot \beta)}{\cot \alpha \cot \beta}$
(b) $a(\tan \beta-\tan \alpha)$
(c) $\frac{a \tan \alpha}{\tan \beta}$
(d) $a(\cot \alpha-\cot \beta)$