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
Answer: C
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