A long, straight cylindrical conductor of radius \(R\) carries a uniform current density \(J\) directed parallel to its central axis. The conductor contains an off-axis cylindrical cavity of radius \(a\) running along its entire length, with central axis parallel to and displaced by a distance \(d\) from the main conductor's central axis. Which of the following expressions correctly gives the magnitude of the magnetic field \(B\) at a point inside the cavity located at a distance \(r\) from the main conductor's central axis?