Explanation

(c) Let radius of curvature of the common internal film surface of the double bubble formed be r’. Then, excess of pressure as
compared to atmosphere inside$\ A\ is\frac{4T}{r_{1\ }}\ $and$\ B\ is\frac{4T}{r_{2}}$
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The pressure difference is
$\frac{4T}{r_{1}} = \frac{4T}{r_{2}} = \frac{4T}{r^{''}}$
$\Longrightarrow \ r^{''} = \frac{r_{1}r_{2}}{r_{2} - r_{1}}$
$Given,\ \ \ r_{1} = r_{2} = r$
$\therefore\ r^{''} = \frac{r_{2}}{0} = \infty$