Let $f ( x , y ) = \sin ( x + y ) + \sin x + \sin y$

Multiple Choice

Let $f ( x , y ) = \sin ( x + y ) + \sin x + \sin y$ . Then f has a relative minimum at

A) $\left( - \frac { 5 \pi } { 3 } , \frac { 5 \pi } { 3 } \right)$
B) $\left( \frac { 5 \pi } { 3 } , \frac { 5 \pi } { 3 } \right)$
C) $\left( \frac { \pi } { 3 } , \frac { \pi } { 3 } \right)$
D) $\left( - \frac { \pi } { 3 } , \frac { \pi } { 3 } \right)$
E) $\left( \frac { 5 \pi } { 3 } , - \frac { 5 \pi } { 3 } \right)$