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

Multiple Choice

Let $f ( x ) = e ^ { x }$ 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) $( - \infty , 0 )$
C) $( 0 , \infty )$
D) $( - \infty , 3 )$
E) $\varnothing$