$CP$ and $CQ$ are tangent from an external point $C$ to a circle with centre $O$. $AB$ is another tangent which touches the circle at $R$. If $CP = 11 \;cm$ and $BR = 4\;cm$, then the length of $BC$ is _________

Question

$\begin{array}{1 1} (a) \;5\;cm \\ (b)\;6\;cm \\(c)\;7\;cm \\ (d)\;8\;cm \end{array} $