A thin sheet of high-permeability iron is inserted between a small bar magnet and a sensor that measures the magnetic field at a fixed point 0.30m from the magnet along the axial line. The sheet fills the entire cross-section between the magnet and the sensor. Which statement best describes how the magnitude of the field measured at the sensor changes after the sheet is inserted?
a) B becomes nearly zero; the iron sheet blocks virtually all magnetic field lines from reaching the sensor.
b) B decreases but remains non-zero because much of the flux is diverted into the high-permeability sheet rather than through the air path to the sensor.
c) B remains essentially unchanged; the sheet has no effect on the external magnetic field.
d) B increases; the sheet reflects and focuses field lines toward the sensor, concentrating the flux.