Two soap bubbles each with radius r₁ and r₂ coalesce in vacuum under isothermal conditions to form a bigger bubble of radius R.Then, R is equal to
a. √r₁²+r₂²
b. √r₁²−r₂²
c. r₁²+r₂²
d. √r₁²+r₂² / 2
d. 2√r₁²+r₂²