4. Which of the following functions is an increasing function? A. $f ( x ) = - x$ B. $f ( x ) = \left( \frac { 2 } { 3 } \right) ^ { x }$ C. $f ( x ) = x ^ { 2 }$ D. $f ( x ) = \sqrt [ 3 ] { x }$
4. Which of the following functions is an increasing function?\\
A. $f ( x ) = - x$\\
B. $f ( x ) = \left( \frac { 2 } { 3 } \right) ^ { x }$\\
C. $f ( x ) = x ^ { 2 }$\\
D. $f ( x ) = \sqrt [ 3 ] { x }$