15. If $\sin 2 x \sin 3 x=\cos 2 x \cos 3 x$, then one value of $\mathrm{x}$ is
(A) $18^{\circ}$;
(B) $30^{\circ}$; (C) $36^{\circ}$;
(D) $45^{\circ}$;
(E) $60^{\circ}$.
A

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