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

Multiple Choice

Let $f ( x ) = \cos x - e ^ { x }$ If $x \in [ 0,2 \pi ]$ then ƒ has a horizontal tangent line for each x in

A) $\left\{ \frac { \pi } { 2 } \right\}$
B) $\{ \pi \}$
C) $\left\{ \frac { 3 \pi } { 2 } \right\}$
D) $\{ 2 \pi \}$
E) $\varnothing$