Let $f(x)=x^3-27x.$ What are the critical number$($s$)?$

a$.x=-3\sqrt[\phantom{2}]{3}$ and $x=3\sqrt[\phantom{2}]{3}$

c$.x=0,x=-3\sqrt[\phantom{2}]{3},$ and $x=3\sqrt[\phantom{2}]{3}$

b$.x=-3$ and $x=3$

d$.x=0,x=-3,$ and $x=3$

a

b

C

d