A spherical drop of water has 1 mm radius. If the surface tension of the water is $50\times {10}^{-3}$ N/m, then the difference of pressure between inside and outside the spherical drop is:

• A

  25 $\mathrm{N}/{\mathrm{m}}^2$

• B

  10000 $\mathrm{N}/{\mathrm{m}}^2$

• C

  100 $\mathrm{N}/{\mathrm{m}}^2$

• D

  50 $\mathrm{N}/{\mathrm{m}}^2$