Three concentric metallic spherical shells `A,B` and `C` of radii `a,b` and `c(a lt blt c)` have surface charge densities `-sigma ,+sigma `and `-sigma` respectively ,the potential of shell `A` is
A. `sigma/epsi_(0) [(b^(2)-c^(2))/b+a]`
B. `sigma/epsi_(0) [(b^(2)-c^(2))/c+a]`
C. `sigma/epsi_(0) [(a^(2)-b^(2))/a+c]`
D. `sigma/epsi_(0) [(a^(2)-b^(2))/b+c]`
