Find the limit$.$

$\underset{\{\begin{array}{ccc}\text{:}[(x,& y)(\frac{81}{2},& \frac{81}{2})]\\ x& y81\end{array}}{lim}\frac{xy-81}{\sqrt[\phantom{2}]{xy}-9}$

A$.0$

B$.18$

C$.9$

D$.$ There is no limit$.$