Find an Equation for the Graph $y = 5 \cos ( 3 x )$

Multiple Choice

Find an equation for the graph.
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A) $y = 5 \cos ( 3 x )$
B) $y = 3 \cos \left( \frac { 1 } { 5 } x \right)$
C) $y = 5 \cos \left( \frac { 1 } { 3 } x \right)$
D) $y = 3 \cos ( 5 x )$