A Conducting Hollow Cylinder (Inner Radius = A,outer Radius = $\oint \mathrm { B } \cdot d \mathbf { s }$

Multiple Choice

A conducting hollow cylinder (inner radius = a,outer radius = b) carries a current of 40 A that is uniformly distributed over the cross section of the conductor.If a = 3.0 mm and b = 6.0 mm,what is the magnitude of the (line) integral $\oint \mathrm { B } \cdot d \mathbf { s }$ around a circular path (radius = 5.0 mm) centered on the axis of the cylinder and in a plane perpendicular to that axis?

A) 50 $\mu$ T . m
B) 30 $\mu$ T . m
C) 22 $\mu$ T. m
D) 37 $\mu$ T . m
E) 47 $\mu$ T . m