\({\rm{\;}}\smallint {\sin ^2}3x\;dx =\)

• A. \({\rm{\;}}\frac{1}{2}\left[ {x - \frac{{\sin 6x}}{6}} \right] + c\)
• B. \({\rm{\;}} - \frac{1}{2}\left[ {x - \frac{{\sin 6x}}{6}} \right] + c\)
• C. \({\rm{\;}} - \frac{1}{2}\left[ {{x^2} = \pi {r^2} - \frac{{\sin 6x}}{6}} \right] + c\)
• D. none