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4. Consider an infinite wire carrying a current I as shown in the figure. The curve is a semicircle of radius a. What is the magnetic field at the center of the semicircle P? μ₀ is the vacuum permeability.

(A) (π + 1)
(B) (π + 2)
(C) (2π + 1)
(D) (2π + 2)
(E) none of the above