Simplify the Expression Algebraically $$\sin ( 7 x + 7 y ) + \sin ( 7 x - 7 y )$$

Multiple Choice

Simplify the expression algebraically.​ $$\sin ( 7 x + 7 y ) + \sin ( 7 x - 7 y )$$ ​

A) $\sin \left( 7 x ^ { 2 } + 7 y ^ { 2 } \right)$
B) $2 \sin 7 x$
C) $\sin \left( 7 x ^ { 2 } - 7 y ^ { 2 } \right)$
D) $\sin 7 x \cos 7 y$
E) $2 \sin 7 x \cos 7 y$