Let $f ( x ) = x - e ^ { x }$

Multiple Choice

Let $f ( x ) = x - e ^ { x }$ Then $f ^ { \prime } ( x ) = 0$ for all x in

A) $( - \infty , \infty )$
B) $( - \infty , 0 )$
C) $( 0 , \infty )$
D) $( - \infty , e )$
E) $\left( - \infty , e ^ { 2 } \right)$