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.

Which wire has the greatest magnitude of the magnetic field on the surface?

a

slope of curve for wire a is greatest.