Find the derivative of the function.

$q=\sqrt[\phantom{2}]{20r-r^7}$

A$)\frac{20-7r^6}{2\sqrt[\phantom{2}]{20r-r^7}}$

B$)\frac{1}{2\sqrt[\phantom{2}]{20-7r^6}}$

C$)\frac{1}{2\sqrt[\phantom{2}]{20r-r^7}}$

D$)\frac{-7r^6}{\sqrt[\phantom{2}]{20r-r^7}}$