A metal sphere of radius rand specific heats 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:

• A

  $\frac{2{\mathrm{\pi }}^2{\mathrm{n}}^2{\mathrm{r}}^2}{5\mathrm{s}}$

• B

  $\frac{{\mathrm{\pi }}^2{\mathrm{n}}^2}{10{\mathrm{r}}^2\mathrm{s}}$

• C

  $\frac{7}{8}{\mathrm{\pi r}}^2{\mathrm{n}}^2\mathrm{s}$

• D

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