9. The length of a longest interval in which the function $3 \sin x - 4 \sin ^ { 3 } x$ is increasing, is (A) $\pi / 3$ (B) $\pi / 2$ (C) $3 \sqcap / 3$ (D) $\Pi$
9. The length of a longest interval in which the function $3 \sin x - 4 \sin ^ { 3 } x$ is increasing, is\\
(A) $\pi / 3$\\
(B) $\pi / 2$\\
(C) $3 \sqcap / 3$\\
(D) $\Pi$\\