If the arcs of the same length in two circles subtend angles of $60^{\large\circ}$ and $75^{\large\circ}$ at their respective centres,find the ratio of their radii

Question

$\begin{array}{1 1}(A)\;2 : 3&(B)\;3 : 2\\(C)\;4 : 5&(D)\;5 : 4\end{array} $