Two spherical soap bubbles of radii `r_1` and `r_2` in vacuume coalesce under isothermal condition. The resulting bubble has radius `R` such that
A. `R=r_(1)+r_(2)`
B. `R= (r_(1)r_(2))/(r_(1)+r_(2))`
C. `R^(2) = r_(1)^(2) + r_(2)^(2)`
D. ` R = (r_(1)+r_(2))/(r_(2))`