Three concentric metallic spherical shells of radii R, 2R, 3R are given charges $$Q_1, Q_2, Q_3$$ respectively. It is found that the surface charge densities on the outer surfaces of the shells are equal. Then, the ratio of the charges given to the shells, $$Q_1 : Q_2 : Q_3$$ is-

A

$$1 : 2 : 3$$

B

$$1 : 3 : 5$$

C

$$1 : 4 : 9$$

D

$$1 : 8 : 18$$