$A$ small bob $A$ of mass $m$ is attached to a massless rigid rod of length $1 \ m$ pivoted at point $P$ and kept at an angle of $60^{\circ}$ with the vertical as shown in the figure. At a distance of $1 \ m$ below point $P$, an identical bob $B$ is kept at rest on a smooth horizontal surface that extends to a circular track of radius $R$ as shown in the figure. If bob $B$ just manages to complete the circular path of radius $R$ up to a point $Q$ after being hit elastically by bob $A$, then the radius $R$ is . . . . . . $m$.

  • A
    $ \frac{3}{5} $
  • B
    $ \frac{1}{5} $
  • C
    $ \frac{2+\sqrt{3}}{5} $
  • D
    $ \frac{2-\sqrt{3}}{5} $

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