On a long horizontally moving belt,a child runs to and fro with a speed $9 \, km \, h^{-1}$ (with respect to the belt) between his father and mother located $50 \, m$ apart on the moving belt. The belt moves with a speed of $4 \, km \, h^{-1}$. For an observer on a stationary platform,the speed of the child running in the direction of motion of the belt is ..... $km \, h^{-1}$.

  • A
    $4$
  • B
    $5$
  • C
    $9$
  • D
    $13$

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