13 An infinitely long cylinder, of radius R, caries a frozen-in magnetization, parallel to the axis, M=ksẑ, where k is a constant and s is the distance from the axis; there is no free current anywhere. Find the magnetic field outside the cylinder (A)0 (B)μ₀ks (C)2πμ₀ks (D)2μ₀ks (E)4πμ₀ks