A bob of mass $m$ is suspended at a point $O$ by a light string of length $l$ and left to perform vertical motion (circular) as shown in figure. Initially, by applying horizontal velocity $v_0$ at the point ' A ', the string becomes slack when, the bob reaches at the point '$D$ '. The ratio of the kinetic energy of the bob at the points B and C is $\underline{\hspace{2cm}}$.