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Find $f'(x)$ when $f(x)=\sin ^{-1} (3x)$

$\cfrac{3}{\sqrt{1-3x^2}}$

$\cfrac{3}{\sqrt{1-9x^2}}$

$\cfrac{1}{\sqrt{9x^2-1}}$

$\cfrac{3}{\sqrt{3x^2-1}}$