4. (5 pts) Fig. 7 shows an infinitely long cylindrical current distribution with radius a and with its axis coincides with the z-axis. The current density J(r) = Josin(2r/a)/r, where Jo is a Vx2 + y2, and the direction of J(r) is in the +z-direction. Which of positive constant and r = the following is the B-field along the x-axis for x 2 a resulted from J(r)? B B B, aj aj ai (A) (B)| (C) (D) (E) (F)| (G) P Fig. 6