Find the Largest Open Interval Where the Function Is Changing $\quad f ( x ) = x ^ { 3 } - 4 x$

Multiple Choice

Find the largest open interval where the function is changing as requested.
-Decreasing $\quad f ( x ) = x ^ { 3 } - 4 x$

A) $\left( - \infty , - \frac { 2 \sqrt { 3 } } { 3 } \right)$
B) $( - \infty , \infty )$
C) $\left( - \frac { 2 \sqrt { 3 } } { 3 } , \frac { 2 \sqrt { 3 } } { 3 } \right)$
D) $\left( \frac { 2 \sqrt { 3 } } { 3 } , \infty \right)$