Let $f ( x ) = x ^ { 3 } - x ^ { 2 } - x + 8$

Multiple Choice

Let $f ( x ) = x ^ { 3 } - x ^ { 2 } - x + 8$ The set of all critical numbers of ƒ is

A) $\left\{ - 1 , - \frac { 1 } { 3 } \right\}$
B) $\left\{ - 1 , \frac { 1 } { 3 } \right\}$
C) $\left\{ 1 , \frac { 1 } { 3 } \right\}$
D) $\left\{ 1 , - \frac { 1 } { 3 } \right\}$
E) $\{ 1 \}$