(21) always ask questions when it is impossible (22) you to answer. My dentists had just pulled out one of my (23) and had told me to rest for a while. I tried (24) something, but my mouth was full of cotton-wool. He knew I collected stamps and asked me whether my collection (收藏) was (25) . He then asked how my sister was and whether I liked my new classmates in London. (26) answer to the questions, I either nodded or made strange noises. At the same time, my tongue was busy (27) for the hole (28) the tooth had been. I suddenly felt very worried, but couldn’t say (29) When the dentist at last removed (拿走) the cotton-wool from my mouth, I was able to tell him that he had pulled out the (30) tooth.