Evaluate the following integral using trigonometric substitution.

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}\frac{dx}{\sqrt[\phantom{2}]{x^2-400}},x>20$

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

A$.x=20sin$

B$.x=20tan$

C$.x=20sec$

Rewrite the given integral using this substitution.

${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}\frac{dx}{\sqrt[\phantom{2}]{x^2-400}}={\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}0d$

$($Type an exact answer.$)$