B is a point situated vertically upwards on A at a distance of a unit. C is a point at a height ‘2a’ units from a horizontal passing through A and the angles of elevation of C from A and B are $\alpha$ and $\beta$respectively, then $\cot \alpha \tan \beta =$  

• A

  1

• B

  2

• C

  $\frac{1}{2}$

• D

  3