The radius of a soap bubble is increased from $\frac{1}{\sqrt{\mathrm{\pi }}}$cm to $\frac{2}{\sqrt{\mathrm{\pi }}}$ cm. If the surface tension of water is 30 dynes per cm, then the work done will be

(a) 180 ergs                          (b) 360 ergs

(c) 720 ergs                          (d) 960 ergs

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