Two Cubic Equations

Well done to Alfie from England who submitted this solution:

Alfie also explained algebraically why the answer is B:

The solutions of $f(-x)=0$ are the negated solutions of $f(x)=0$ (since if $a$ is a solution of $f(x)=0,$ then $f(a)=0$ and so $f(-(-a))=0$ which means $-a$ is a solution of $f(-x)=0$).