AMC 12 2005b Problem 6

In $\triangle ABC$, we have $AC=BC=7$ and $AB=2$. Suppose that $D$ is a point on line $AB$ such that $B$ lies between $A$ and $D$ and $CD=8$. What is $BD$?

$\mathrm{(A)}\ 3 \mathrm{(B)}\ 2\sqrt{3} \mathrm{(C)}\ 4 \mathrm{(D)}\ 5 \mathrm{(E)}\ 4\sqrt{2}$