A soap bubble has radius r. The work done in increasing its radius to three times its original radius, without any rise of temperature, is (Given: surface tension of soap solution is T)

• A

  $12{\mathrm{\pi r}}^2\mathrm{T}$

• B

  $16{\mathrm{\pi r}}^2\mathrm{T}$

• C

  $64{\mathrm{\pi r}}^2\mathrm{T}$

• D

  $48{\mathrm{\pi r}}^2\mathrm{T}$