6. Let $x$ be an acute angle, and satisfy $\sin x=3 \cos x$. Then $\sin x \cos x$ equals ( ).
(A) $\frac{1}{6}$
(B) $\frac{1}{5}$
(C) $\frac{2}{9}$
(i) $\frac{3}{10}$
6. D

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