The set of points where the function $f(x)=\sqrt{1-e^{-x^2}}$ is differentiable is

Question

$\begin{array}{1 1}(a)\;(-\infty,\infty)&(b)\;(-\infty,0)\cup (0,\infty)\\(c)\;(-1,\infty)&(d)\;\text{None of these}\end{array}$