Let $f:R\rightarrow R$ be a function defined by $f(x)=max(x,x^3)$.The set of all points where $f(x)$ is not differentiable

Question

$(a)\;\{-1,1\}\qquad(b)\;\{-1,0\}\qquad(c)\{0,1\}\qquad(d)\;\{-1,0,1\}$