An Acoustic Beat Produces a Combined Sound Wave Represented by the Function

Multiple Choice

An acoustic beat produces a combined sound wave represented by the function $f(x) =\cos (2 \pi \cdot 41 x) +\cos (2 \pi \cdot 53 x)$ .

A) $f(x) =-2 \cos (2 \pi \cdot 47 x) \cos (2 \pi \cdot 6 x)$
B) $f(x) =2 \sin (2 \pi \cdot 47 x) \cos (2 \pi \cdot 6 x)$
C) $f(x) =2 \cos (2 \pi \cdot 47 x) \cos (2 \pi \cdot 6 x)$
D) $f(x) =2 \cos (2 \pi \cdot 47 x) \sin (2 \pi \cdot 6 x)$