ABCD is a trapezium such that $BC\parallel AD$ and $AB=4$ cm.If the diagonals AC and BD intersect at O,such that $\large\frac{AO}{OC}=\frac{DO}{OB}=\frac{1}{2}$,then $BC$=

Question

$\begin{array}{1 1}7\;cm\\8\;cm\\9\;cm\\6\;cm\end{array}$