If the angles of elevation of the top of a tower from three collinear points A,B and C on a line leading to the foot of the tower, are $30^{\circ}, 45^{\circ}$ and $60^{\circ}$ respectively , the the ratio , $AB:BC$ is

Question

$\begin{array}{1 1} \sqrt 3 : \sqrt 2 \\ 1 : \sqrt 3 \\ 2:3 \\ \sqrt 3 :1 \end{array} $