A student wants to test whether a steel rod retains magnetism after it has been placed in a strong magnetic field. Which method gives the clearest evidence?
Remove the field, then test whether the rod repels one pole of a compass.
Keep the field present, then test whether a magnet attracts both rod ends.
Keep the field present, then test whether the rod attracts an iron nail.
Remove the field, then test whether a magnet attracts one end of the rod.