Let $f ( x ) = 6 x - e ^ { x }$

Multiple Choice

Let $f ( x ) = 6 x - e ^ { x }$ Then which of the following is the largest interval, of those listed, for which $f ^ { \prime } ( x ) > 0$ for all x in that interval?

A) $( - \infty , \infty )$
B) $( - \infty , \ln 6 )$
C) $( \ln 6 , \infty )$
D) $( - \infty , - \ln 6 )$
E) $( - \infty , - \ln 6 )$ .