设 $f(x)=\left\{\begin{array}{cc}x^2, & x \leqslant 0, \\ x^2+x, & x>0 .\end{array}\right.$ 则（ ）
A
$f(-x)=\left\{\begin{array}{cc}-x^2, & x \leqslant 0, \\ -\left(x^2+x\right), & x>0 .\end{array}\right.$
B
$f(-x)=\left\{\begin{array}{cc}-\left(x^2+x\right), & x < 0, \\ -x^2, & x \geqslant 0 .\end{array}\right.$
C
$f(-x)=\left\{\begin{array}{cc}x^2, & x \leqslant 0, \\ x^2-x, & x>0 .\end{array}\right.$
D
$f(-x)=\left\{\begin{array}{cc}x^2-x, & x < 0, \\ x^2, & x \geqslant 0 .\end{array}\right.$
E
F