A soap bubble of radius r is blown up to from a bubble of radius 2r under isothermal conditions. If T is the surface tension of soap solution, the energy spent in the blowing :

Question

$$\begin {array} {1 1} (1)\;3 \pi T r^2 & \quad (2)\;6 \pi Tr^2 \\ (3)\;12 \pi T r^2 & \quad (4)\;24 \pi T r^2 \end {array}$$