$A$ particle is released on a vertical smooth semicircular track from point $X$ so that $OX$ makes an angle $\theta$ from the vertical (see figure). The normal reaction of the track on the particle vanishes at point $Y$ where $OY$ makes an angle $\phi$ with the horizontal. Then

• A
  $\sin \phi = \cos \theta$
• B
  $\sin \phi = \frac{1}{2} \cos \theta$
• C
  $\sin \phi = \frac{2}{3} \cos \theta$
• D
  $\sin \phi = \frac{3}{4} \cos \theta$