Where are the critical points of the function $f(x)=\frac{1}{27}{x}^3-49x?$

$x=0$

$x=-7,x=7$

$x=-7\sqrt[\phantom{2}]{27},x=7\sqrt[\phantom{2}]{27},x=0$

$x=-7\sqrt[\phantom{2}]{27},x=7\sqrt[\phantom{2}]{27}$

$x=-21,x=21$