The upper half of an inclined plane with inclination $\phi$ is perfectly smooth, while the  lower half is rough. $A$ body starting from rest at the top will again come to rest at the  bottom, if the coefficient of friction for the lower half is given by-

  • A

    $2 \sin \phi$

  • B

    $2 \cos \phi$

  • C

    $2 \tan \phi$

  • D

    $ \tan \phi$

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