Solve the Problem $y = 9 ( \cos 2 \pi x \cos b t + \sin 2 \pi x \sin b t )$

Solve the problem.
-A vibrating wire has a vertical displacement given by $y = 9 ( \cos 2 \pi x \cos b t + \sin 2 \pi x \sin b t )$ . Assume $b$ and $t$ are constants. Write $y$ as a cosine function of $x$ .

A) $\mathrm { y } = \cos ( 2 \pi \mathrm { x } - \mathrm { bt } )$
B) $y = 9 \cos ( 2 \pi x + b t )$
C) $y = 9 \cos ( 2 \pi x - b t )$
D) $y = 18 \cos ( 2 \pi x + b t )$