An annular disc of radius ${\mathrm{r}}_1 {=}_{ }$10 cm and ${\mathrm{r}}_2$ = 5 cm is placed on a water surface. Find the surface tension force on the disc if we want to pull it from water surface. Take surface tension $\mathrm{\sigma } = 7\times {10}^{-3} \mathrm{N}/\mathrm{m}, \mathrm{g}=10 \mathrm{m}/{\mathrm{s}}^2$

• A

  6782.4 dyne

• B

  67.82 dyne

• C

  678.24 dyne

• D

  none of these