函数 $f\left(x\right)=x^2\left(x\le 0\right)$的反函数是（）

A．

$f^{-1}\left(x\right)=\sqrt{x}\left(x\ge 0\right)$

B．

$f^{-1}\left(x\right)=-\sqrt{x}\left(x\ge 0\right)$

C．

$f^{-1}\left(x\right)=-\sqrt{-x}\left(x\le 0\right)$

D．

$f^{-1}\left(x\right)=-x^2\left(x\le 0\right)$