Evaluate the integral: ${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}}\frac{15\sqrt[\phantom{2}]{x^2-64}}{x^4}dx$

$($A$)$ Which trig substitution is correct for this integral?

$x=8sin()$

$x=8sec()$

$x=64sec()$

$x=15sin()$

$x=64tan()$

$x=8tan()$

$($B$)$ Which integral do you obtain after substituting for $x$ and simplifying?

Note: to enter $,$ type the word theta.

$[$:${\displaystyle \underset{\phantom{}}{\overset{\phantom{}}{}}},$