The figure shows a cross section of three parallel wires each carrying a current of 15 A. The currents in wires A and C are out of the paper, while that in wire B is into the paper. If the distance R = 5.0 mm, what is the magnitude of the force on a 4.0-m length of wire C?
A. 90 mN
B. 54 mN
C. 30 mN
D. 18 mN
E. 36 mN
Answer: D
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