A student carries out an experiment to find out which of four metal rods makes the best permanent magnet.
The rod is attached to the south pole of a permanent magnet, and then paper clips are attached to the rod. The number of paper clips that can be held by the rod is counted.
The permanent magnet is then removed from the rod, and the number of paper clips that remain attached are counted.
The results are summarised in the table below.
Which of the rods becomes the strongest permanent magnet?