An infinitely long straight conducting cylinder of radius 4R lies along the z-axis. The current density in the cylinder is J along the -z axis and uniform over the cross-section. Find the magnitude and direction of the magnetic field at point A.
Select one:
a.??JR, +x-axis
b.??JR, -x-axis
c.2??JR, +y-axis
d.??JR, +y-axis
e.2??JR, -x-axis
f.??JR, -y-axis
g.2??JR, -y-axis
h.2??JR, +x-axis