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 , then the ratio , $AB:BC$ , is :

Question

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