A pole stands vertically on a road, which goes in the north-south direction. P, Q are two points towards the north of the pole, such that PQ = b, and the angles of elevation of the top of the pole at P, Q are α, β respectively. Then the height of the pole is:

$\frac{b}{\tan \beta - \tan \alpha}$ $\frac{b \tan \alpha}{\tan \beta}$ $\frac{b}{\tan \beta + \tan \alpha}$ $\frac{b}{\cot \beta - \cot \alpha}$

$\frac{b}{\cot \beta - \cot \alpha}$