III．阅读理解． (20分) A Dentists always ask questions when it is impossible for you to answer．One day ,my dentist had just pulled out one of my teeth and asked me to sit in a chair to rest for a while． I tried to say something ,but my mouth was full of cotton-wool(药棉)． He knew I collected stamps and asked me whether I got any new recently． He then asked me how my brother was and whether I liked my new job． As I could not speak ,l either nodded or made strange noises to answer these questions． At the same time ,l tred hard to use my tonguec舌头） to find the hole(洞) in the place of my bad tooth． Suddenly I felt very worried because the bad tooth was still there ,but I could not say any- thing． At last the dentist pulled the cotton-wool out of my mouth ,and I was able to tell lum that he had pulled out the wrong tooth． ( )21． Why did the dentist ask me to rest for a while?