In a coaxial straight cable, the inner conductor and the outer conductor have equal currents in opposite directions. The magnetic induction is zero -

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 

Toroid is a solenoid of