Three concentric metallic spherical shells of radii R , 2R , 3R , are given charges Q1 , Q2 , 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 [2009]
a. 1:2:3
b. 1:3:5
c. 1:4:9
d. 1:8:18