Write the Expression as the Sine, Cosine, or Tangent of an Angle

Multiple Choice

Write the expression as the sine, cosine, or tangent of an angle.
- $\cos \frac { \pi } { 7 } \cos x - \sin \frac { \pi } { 7 } \sin x$

A) $\cos \left( \frac { \pi } { 7 } + x \right)$
B) $\sin \left( \frac { \pi } { 7 } - x \right)$
C) $\sin \left( \frac { \pi } { 7 } + x \right)$
D) $\cos \left( \frac { \pi } { 7 } - x \right)$