The Largest Set on Which the Function $f ( x ) = 5 ^ { - x }$

Multiple Choice

The largest set on which the function $f ( x ) = 5 ^ { - x }$ is decreasing is

A) $( - \infty , \infty )$
B) $( - \infty , - 5 )$
C) $( - 5 , \infty )$
D) $( - 5,5 )$
E) $( 5 , \infty )$