In the 1950s,a family who owned a farm near Beulah,Michigan kept a bull chained to an elm  .The bull paced around the tree,dragging the heavy iron chain,which led to a groove   in the bark.The groove deepened over the years,though for whatever reason,it did not kill the tree. After some years,the family took their bull away.They cut the chain,leaving the loop   around the tree and one link hanging down. Then one year,agricultural disaster struck Michigan in the form of Dutch Elm Disease.All of the elms lining the road leading to the farm became infected and died.Everyone thought that the old elm would be next. The farm's owners considered doing the safe thing:pulling it out and cutting it up into firewood before it died.But they simply could not bring themselves to do it.It was as if the old tree had become a family friend.So they decided to _ . Amazingly,the tree did not die.Nobody could understand why it was the only elm that was still standing in the country! Plant pathologists  from Michigan State University came out to observe the tree.They observed the scar left by the iron chain,now almost completely covered by bark.The plant experts decided that it was the chain that saved the elm's life.They reasoned that the tree must have absorbed so much iron from the chain that it became immune   to the fungus  . It's said that what doesn't kill you will make you stronger.Or,as Ernest Hemimgway put it,"Life breaks us all,but afterwards,many of us are strongest at the broken places." From the passage we can learn that the old elm was saved by  _  .
the groove in the bark the fungus in the tree the iron remaining in it its own immune system