Suppose $\sin^2x\sin 3x=\sum\limits_{m=0}^{n}C_m\cos mx$ is an identity in $x$ where $C_0,C_1.......C_n$ are constants and $C_n\neq 0$ then the value of n is

Question

$(a)\;6\qquad(b)\;5\qquad(c)\;4\qquad(d)\;3$