The ratio in which the line segment joining points $A(a_1,b_1)$ and $B(a_2,b_2)$ is divided by y-axis is ________

Question

$\begin{array}{1 1}(A) \;-a_1 :a_2 \\(B)\;a_1:a_2 \\(C)\;b_1 :b_2 \\(D)\;-b_1 : b_2 \end{array}$