Let $f(\theta ) = \sin \theta (\sin \theta + \sin 3\theta )$, then $f(\theta )$

  • [IIT 2000]
  • A

    $ \ge 0$ only when $\theta \ge 0$

  • B

    $ \le 0$ for all real $\theta $

  • C

    $ \ge 0$ for all real $\theta $

  • D

    $ \le 0$ only when $\theta \le 0$

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  • [JEE MAIN 2021]

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