Three concentric spherical metallic shells A , B and C of radii a , b and c $\;(a < b < c)\;$ have charge densities $\;\sigma , - \sigma \; and \; \sigma\;$ respectively . If the shells A and C are at the same potential then the relation between a , b and c is

Question

$(a)\;a+b+c=0\qquad(b)\;a+c=b\qquad(c)\;a+b=c\qquad(d)\;2a+b=c$