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

• Correct answer
  A

  $\frac{2{\mathrm{\pi }}^2{n}^2{r}^2}{5S}$
• B

  $\frac{{\mathrm{\pi }}^2{n}^2}{10r^{2}S}$
• C

  $\frac{7}{8}\mathrm{ }\mathrm{\pi }{r}^2{n}^{2}S$
• D

  $\frac{5{\left(\mathrm{\pi }rn\right)}^2}{14S}$

Hence, the correct answer is option: 'A'.