Three concentric metallic spherical shells of radii R, 2R and 3R are given charges Q1, Q2 and Q3 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, Q1 :Q2 : Q3 is
(A) 1:2:3
(B) 1:3:5
(C) 1:4:9
(D) 1:8:18
