For the following $f(x)$, determine ALL the $x$ value/values where $f(x)$ is
not differentiable:
$f(x) = \begin{cases} 0, & x \le 0 \\ 5 - x, & 0 < x < 4 \\ \frac{1}{(5 - x)}, & x \ge 4 \end{cases}$
(a) $x = 0$
(b) $x = 0, 4$
(c) $x = 0, 4, 5$
(d) None