The angle of elevation of the top of a tower standing on a horizontal plane from a point A is α. After walking a distance 'd' towards the foot of the tower the angle of elevation is found to be β. The height of the tower is

• A. $\frac{d}{{\cot \alpha+ \cot \beta }}$$
• B. $\frac{d}{{\cot \alpha- \cot \beta }}$$
• C. $\frac{d}{{\tan \beta- \operatorname{tant} \alpha }}$$
• D. $\frac{d}{{\tan \beta+ \operatorname{tant} \alpha }}$$