Find the given limit$.$

$\underset{x{0}^{}}{lim}\sqrt[\phantom{2}]{x^2-64}$

Select the correct choice below and, If necessary, fill in the answer box to complete your choice.

A$.\underset{x{8}^{}}{lim}\sqrt[\phantom{2}]{x^2-64}=$

B$.$ The limit does not exist and is not $$ or $-$