Let $f ( x ) = \frac { 1 } { 4 } x ^ { 4 } - \frac { 1 } { 3 } x ^ { 3 } - 3 x ^ { 2 } + 1$

Multiple Choice

Let $f ( x ) = \frac { 1 } { 4 } x ^ { 4 } - \frac { 1 } { 3 } x ^ { 3 } - 3 x ^ { 2 } + 1$ The set of all critical numbers of ƒ is

A) $\{ - 2,0 \}$
B) $\{ 2,3 \}$
C) $\{ 0 , - 3 \}$
D) $\{ - 2,0,3 \}$
E) $\{ - 2,0 \}$ .