A Long Wire Is Known to Have a Radius Greater

Multiple Choice

A long wire is known to have a radius greater than 4.0 mm and to carry a current uniformly distributed over its cross section. If the magnitude of the magnetic field is 0.285 mT at a point 4.0 mm from the axis of the wire and 0.200 mT at a point 10 mm from the axis, what is the radius of the wire?

A) 4.6 mm
B) 7.1 mm
C) 5.3 mm
D) 12 mm
E) 10 mm