$A$ metal sphere of radius $r$ and specific heat $S$ is rotated about an axis passing through its centre at a speed of $f$ rotations per second. It is suddenly stopped,and $50\%$ of its rotational kinetic energy is used in increasing its temperature. The rise in temperature of the sphere is

  • A
    $\frac{2\pi^2 f^2 r^2}{5S}$
  • B
    $\frac{\pi^2 f^2}{10r^2 S}$
  • C
    $\frac{7}{8}\pi r^2 f^2 S$
  • D
    $\frac{5(\pi rf)^2}{14S}$

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