doubt

electric field at A and B are 0

i supppose A ,B are on the conductor surface arent they ?

i am not very sure but feel

σ = q/4Î r² where r =a,b for A and B

http://targetiit.com/iit_jee_forum/posts/cavity_1892.html

i once postd this qn,but the fiitjee 's anss gave a contradiction.

what about charge densities????????????????