A. $\int \cos 3 x d x=3 \sin 3 x+C$.

B. $\int \cos 3 x \mathrm{~d} x=\frac{\sin 3 x}{3}+C$.

C. $\int \cos 3 x \mathrm{~d} x=\sin 3 x+C$.

D. $\int \cos 3 x \mathrm{~d} x=-\frac{\sin 3 x}{3}+C$.