If the angle 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} \sqrt 3 :1 \\ \sqrt 3 :\sqrt 2 \\ 1: \sqrt 3 \\ 2:3 \end{array} $