Find the Critical Number(s), If Any, of the Function $f ( x ) = x ^ { 3 } - 21 x + 4$

Multiple Choice

Find the critical number(s) , if any, of the function $f ( x ) = x ^ { 3 } - 21 x + 4$ .

A) 4
B) $- 4$
C) $\pm \sqrt { 7 }$
D) $\pm 7$