[" A current flows in an infinitely long wire with cross-section in the form of a semicircular nig of radius "],[" R.The magnitude of the magnetic induction along its axis is: "],[[" (1) "(mu_(0)I)/(pi^(2)R)," (2) "(mu_(0)I)/(2 pi^(2)R)," (3) "(mu_(0)I)/(2 pi R)," (4) "(mu_(0)I)/(4 pi R)]]