15. We find that N current loops are coplanar and coaxial. The first has radius a and current I. The second has radius 2a and current 2I, and the pattern is repeated up to the Nth, which has radius Na and current NI. The current in each loop is counterclockwise as seen from above. The magnitude of the magnetic field at the center of the loops is (A) μ₀I/2Na (B) μ₀I/Na (C) μ₀I/2a (D) μ₀NI/2a (E) μ₀NI/a