JEE Math Practice Question

Let f : R $$ \to $$ R be a function defined by f(x) = max {x, x2}. Let S denote the set of all points in R, where f is not differentiable. Then :

1. A.{0, 1}
2. B.{0}
3. C.$$\phi $$(an empty set)
4. D.{1}