1. 单句改错， 每个句子有一处错误，找出并改正

1. （1） Sure, I could have asked her, but something prevented me from do so.
   
3. （2） He was writing a report while one of his friends called on him.
   
5. （3） What is the most important is that we should apply theory for practice.
   
7. （4） There is no doubt whether it is her achievements that have won her the Nobel Prize.
   
9. （5） Though warning again and again, the young man still drove after drinking, leading to his being fined.
   
11. （6） What the volunteers did made great difference to the children in the small village.
    
13. （7） Mr.Li fell ill, so his colleague attended the conference in the place of him.
    
15. （8） I was about to informing you of the sudden change when an unexpected visitor came.
    
17. （9） The little boy won't go to sleep if his mother tells him a story.
    
19. （10） A man immediately rushed to the girl give her first aid and I joined in without hesitation.