12. A toroid has a donut shape. It is circular in cross section, with an inner radius of 4.0 cm and an outer radius of of 6.0 cm (i.e. the diameter of the circular cross section is 2.0 cm), and is wound with 400 turns of wire. What current must be set up in the windings to attain an average magnetic around 4.0 mT in the toroid? (A) 1.5 (B) 2.0 (C) 2.5 (D) 3.0 [A]