Two radii of two concentric circles are $13\;cm$ and $8\;cm$ . AB is a diameter of the bigger circle BD is a tangent to the smaller circle touching it at D. Then the length of AD is ________

Question

$\begin{array}{1 1} 19 cm \\ 19\;\sqrt 2\;cm \\19 \sqrt 3\;cm \\ \text{none of the above} \end{array}$