If the angles of elevation of a tower from two points distant `a` and `b` `(a > b)` from its foot and in the same straight line with it are `30^@` and `60^@` , then the height of the tower is `(a) sqrt(a+b)` (b) `sqrt(a b)` (c) `sqrt(a-b)` (d) `sqrt(a/b)`