21.A coaxial cable consists of an inner and an outer cylindrical thin shell of radii R₁ and R₂, respectively. A current of I amp flows along each shell in opposite directions. Which statements are correct?
(A) The magnetic field B at r ＞ Rₒ is 0.
(B) The magnetic field B at r ＜ R₁ is 0.
(C) The magnetic energy stored in the region R₁ ＜ r ＜ R₂ per unit axial length is 0.
(D) The magnetic energy stored in the region R₁ ＜ r ＜ R₂ per unit axial length is .
(E) A nonzero magnetic pressure exerts on the outer shell.