The common radius of curvature ' $r^{\prime}$, when two soap bubbles with radii $r_{1}$ and $r_{2}\left(r_{1}>r_{2}\right)$ come in contact, is:
(a) $r=\frac{r_{1}+r_{2}}{2}$
(b) $r=\frac{r_{1} r_{2}}{r_{1}-r_{2}}$
(c) $r=\frac{r_{1} r_{2}}{r_{1}+r_{2}}$
(d) $r=\sqrt{r_{1} r_{2}}$