The radius of the circular plates of a parallel plate condenser is ‘r’. Air is there as the dielectric. The distance between the plates if its capacitance is equal to that of an isolated sphere of radius r' is

• A

  $\frac{{\mathrm{r}}^2}{4\mathrm{r}\text{'}}$

• B

  $\frac{{\mathrm{r}}^2}{\mathrm{r}\text{'}}$

• C

  $\frac{\mathrm{r}}{\mathrm{r}\text{'}}$

• D

  $\frac{{\mathrm{r}}^2}{4}$