Mark Twain was a great American writer .One day he went to a city

Mark Twain was a great American writer .One day he went to a city
Mark Twain was an American writer.One day he went to a city by train.He wanted to see one of his friends there.He was a very busy man.He usually forgot something.
When he was in the train,the conductor asked him for his ticket.Mark Twain looked for his ticket here and there,but he couldn’t find it.The conductor knew Mark Twain was a famous writer.She said,“Show me your ticket on your way back.And if you can’t find it,it doesn’t matter.”
“Oh,but it does,” said Mark Twain.“I must find the ticket,if I can’t find it,how can I know where I am going.”的中文翻译且回答一个问题：Mark Twain didn't show his ticket because he couldn't( )it （供选择的答案有：buy \bring\find\know)