Case : Read the passage and answer the question(s).

Magnetic Field of a hollow wire with a constant current is homogenous. Curves in the graph shown, give, as functions of distance r, the magnitude B of the magnetic field inside and outside four long wires a, b, c and d carrying currents that are uniformly distributed across the cross sections of the wires. Overlapping portions of the plots are indicated by double labels.

In a coaxial, straight cable, the central conductor and the outer conductor carry equal currents in opposite direction. The magnetic field is then zero : 

inside the inner conductor

There is no current flowing if an Amperian loop is constructed just inside the inner cable.