A Long Straight Wire of Radius R Carries a Current $\mu$

Multiple Choice

A long straight wire of radius R carries a current density J = kr A/m² where k is a constant.The magnetic field for r < R is (Hint: Current enclosed .)

A) $\mu$ ₀kr²/2
B) $\mu$ ₀kr²/3
C) 2 $\pi\mu$ ₀kr
D) 2 $\pi\mu$ ₀kr/2
E) none of the above