Find the points of discontinuity, and determine whether the discontinuities are removable.
(a) $f(x)=\frac{x^{2}-4}{x^{3}-8}$
(b) $f(x)=\left\{\begin{array}{ll}2 x-3, & x \leq 2 \\ x^{2}, & x>2\end{array}\right.$
(c) $f(x)=\left\{\begin{array}{ll}3 x^{2}+5, & x \neq 1 \\ 6, & x=1\end{array}\right.$