A young father was visiting an old neighbor. They were standing in the old man's garden,and talking about children. The young man said,"How strict should parents be with their children?"
The old man pointed to a string between a big strong tree and a thin young one."Please untie that string," he said.The young man untied it,and the young tree bent over to one side. "Now tie it again,please," said the old man,"but first pull the string tight so that the young tree is straight again."
The young man did so.Then the old man said,"There,it is the same with children.You must be strict with them,but sometimes you must untie the string to know how they are getting on. If they are not yet able to stand alone,you must tie the string tight again.But when you find that they are ready to stand alone,you can take the string away."
When can the string be taken away?  _  .
When the old man has left After you have untied it When the young man has untied it next time When the young tree grows strong enough