Once there lived an old man in a town. He always forgot a lot of things. So his wife always had to say to him，“Don’t forget this.”
One day he went on a long way alone. Before he left home，his wife said，“Now you have all these things. You need them on your way. Take care of your things on the way.” He went to the station. He bought a ticket and got on the train with it.
About an hour later，the conductor began to see the tickets. He came to the old man and said，“Will you please show me your ticket?” The old man looked for his ticket in all his pockets but he could not find it. He was very worried.“I can’t find my ticket. I really bought a ticket before I got on the train ”said the old man.
“I think you are right. I believe you bought a ticket. All right，you don’t have to buy another ticket ”said the conductor kindly. But the old man still(仍然) looked worried and said sadly，“You don’t know why I’m worried. If I don’t find my ticket，I can’t remember my station. Where am I going? ”

长江中下游地区最突出的自然灾害是（    ）