第2008题：积化和差公式

计算不定积分 $\int \cos (x) \cos (2x) \cos (3x) dx$ .

A. $-\dfrac{\cos 2x}{4}-\dfrac{\cos 4x}{8}$ $-\dfrac{\cos 6x}{12}-\dfrac{x}{3}+C$

B. $-\dfrac{\cos 2x}{8}-\dfrac{\cos 4x}{16}$ $-\dfrac{\cos 6x}{24}-\dfrac{x}{4}+C$

C. $\dfrac{\sin 2x}{4}+\dfrac{\sin 4x}{8}$ $+\dfrac{\sin 6x}{12}+\dfrac{x}{3}+C$

D. $\dfrac{\sin 2x}{8}+\dfrac{\sin 4x}{16}$ +$\dfrac{\sin 6x}{24}+\dfrac{x}{4}+C$