The set of points where $f\left( x \right) = {x \over {1 + \left| x \right|}}$ is differentiable is
$\left( { - \infty ,0} \right) \cup \left( {0,\infty } \right)$
$\left( { - \infty ,1} \right) \cup \left( { - 1,\infty } \right)$
$\left( { - \infty ,\infty } \right)$
$\left( {0,\infty } \right)$
