JEE Advanced 2020 Full Test 1 Paper 1

A long uniform solid cylinder of radius r and resistivity ρ carries current ' l ' along its length which is uniformly distributed throughout its cross sectional area. Two long hollow cylinder B and C of radii 8r and 64r surrounds the solid cylinder. The surface of all cylinder A,B and C are perfectly black. At equilibrium, the temperature of B is 400K.
[ Take ‌

σr3

ρ

=‌

10−8

π2

S.I unit, σ= Stefan's constant ]. Then, at equilibrium.