Mark against the correct answer in each of the following:

Mark $(\sqrt{ })$ against the correct answer in each of the following:

$\int \cos 3 x d x=?$

A. $-\frac{1}{3} \sin 3 x+C$

B. $\frac{1}{3} \sin 3 x+C$

C. $3 \sin 3 x+C$

D. $-3 \sin 3 x+c$