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.

What is true about magnetic field if a direct current I flows along the length of an infinitely long straight thin walled pipe?

zero at any point inside the pipe

The amperian loop imagined inside the pipe will not enclose any current and hence using ampere circuital law, magnetic field is zero.