$[12]$

$4.$ Evaluate each limit$.$ For nonexistent limits$,$ give the best answer from among $,-,$ DNE. As always, show supporting work and$/$or explanation on these.

a$)\underset{x\frac{1}{(2{)}^{-}}}{lim}\frac{2x-5}{(2x-1{)}^3}$

b$)\underset{x-}{lim}\frac{80-10x+0\mathrm{.}5x^4}{50+20x-6x^3}$

c$)\underset{x}{lim}\frac{2x}{\sqrt[\phantom{2}]{5x^2+8}}$