The sum of angles of elevation of the top of tower from two points distant a and b from the base and in the same straight line with it is $90^{\circ}$. Then the height of the tower is :

Question

\[\begin {array} {1 1} (1)\;a^2b & \quad (2)\;ab^2 \\ (3)\;\sqrt {ab} & \quad (4)\;ab \end {array}\]