Given $f(x)=\frac{1}{\sqrt[\phantom{2}]{1-x^7}}.$ Then $f^{\text{'}}(x)=$

$($a$)f^{\text{'}}(x)=\frac{1}{\sqrt[\phantom{2}]{1-7x^6}}$

$($b$)f^{\text{'}}(x)=\frac{7x^6}{2\sqrt[\phantom{2}]{(1-x^7{)}^3}}$

$($c$)f^{\text{'}}(x)=\frac{-1}{\sqrt[2]{(7x^6{)}^3}}$

$($d$)f^{\text{'}}(x)=\frac{7x^6}{2\sqrt[\phantom{2}]{1-x^7}}$

a

c

d

b