A charged spherical capacitor consists of two concentric spherical shells of radii $$a$$ and $$b$$, where $$b>a$$. Half of the stored electrical energy of this system lies within a spherical region of radius $$r$$ if

A

$$r=\sqrt{ab}$$

B

$$r=\frac{a+b}{2}$$

C

$$r=\frac{2ab}{a+b}$$

D

$$r=\frac{a^2+b^2}{a+b}$$