Evaluate the following integral using trigonometric substitution.

${\displaystyle \underset{9}{\overset{18}{}}}\sqrt[\phantom{2}]{324-x^2}dx$

What substitution will be the most helpful for evaluating this integral?

A$.x=18sin$

B$.x=18tan$

C$.x=18sec$

Rewrite the given integral using this substitution.

${\displaystyle \underset{9}{\overset{18}{}}}\sqrt[\phantom{2}]{324-x^2}dx={\displaystyle \underset{\frac{}{6}}{\overset{\phantom{}}{}}}d$

$($Type exact answers.$)$

Evaluate the integral.

${\displaystyle \underset{9}{\overset{18}{}}}\sqrt[\phantom{2}]{324-x^2}dx=,(Typean$ exact answer.$)$