函数 $y=\left\{\begin{array}{l}2x,x\ge 0\\ -x^2,x<0\end{array}\right.$;的反函数是（）

A．

$y=\left\{\begin{array}{l}\frac{x}{2},x\ge 0\\ \sqrt{-x},x<0\end{array}\right.$

B．

$y=\left\{\begin{array}{l}2x,x\ge 0\\ \sqrt{-x},x<0\end{array}\right.$

C．

$y=\left\{\begin{array}{l}\frac{x}{2},x\ge 0\\ -\sqrt{-x},x<0\end{array}\right.$

D．

$y=\left\{\begin{array}{l}2x,x\ge 0\\ -\sqrt{-x},x<0\end{array}\right.$