The Function Graphed Is of the Form Y = a Sin

Multiple Choice

The function graphed is of the form y = a sin bx or y = a cos bx, where b > 0. Determine the equation of the graph.
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A) $y = 2 \cos ( 3 x )$
B) $y = - 2 \cos ( 3 x )$
C) $y = - 2 \cos \left( \frac { 1 } { 3 } x \right)$
D) $y = - 2 \sin ( 3 x )$