A coaxial cable is made up of two conductors. The inner conductor is solid and is of radius $R_1$ and the outer conductor is hollow of inner radius $R_2$ and outer radius $R_3.$ The space between the conductors is filled with air. The inner and outer conductors are carrying currents of equal magnitudes and in opposite directions. Then, the variation of magnetic field with distance from the axis is best plotted as 

Figure shows the cross-sectional view of the hollow cylindrical conductor with inner radius ' $R$ ' and outer radius '$2R$', cylinder carrying uniformly distributed current along its axis. The magnetic induction at point ' $P$ ' at a distance $\frac{3R}{2}$ from the axis of the cylinder will be

From a cylinder of radius $R,$ a cylinder of radius $R/2$ is removed, as shown. Current flowing in the remaining cylinder is $I$. Magnetic field strength is