Let $f ( x ) = 2 \cos x + x$

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

A) $\left\{ \frac { \pi } { 6 } , \frac { 5 \pi } { 6 } \right\}$
B) $\left\{ \frac { 5 \pi } { 6 } , \frac { 7 \pi } { 6 } \right\}$
C) $\left\{ \frac { \pi } { 6 } , \frac { 11 \pi } { 6 } \right\}$
D) $\left\{ \frac { 5 \pi } { 6 } , \frac { 11 \pi } { 6 } \right\}$
E) $\left\{ \frac { 7 \pi } { 6 } , \frac { 11 \pi } { 6 } \right\}$