Mark against the correct answer in the following:

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

$\int_{0}^{\pi / 2} \sin x \sin 2 x d x=?$

A. $\frac{2}{3}$

B. $\frac{3}{4}$

C. $\frac{5}{6}$

D. $\frac{3}{5}$