Find an Equation for the Graph $y = 2 \cos \frac { 1 } { 3 } x$

Multiple Choice

Find an equation for the graph.
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A) $y = 2 \cos \frac { 1 } { 3 } x$
B) $y = 2 \cos 3 x$
C) $y = 3 \cos \frac { 1 } { 2 } x$
D) $y = 3 \cos 2 x$