A、He has had thirteen decayed teeth. B、He doesn’t have a single decayed tooth. C、He has fewer decayed teeth than other people of

How many teeth have you had filled in the past two years? If you follow the advice of Dr. Faustick, you may be able to reduce the number of your visits to a dentist. Dr. Faustick conducted a two-year survey to find out how to prevent or reduce dental decay. 946 students took part in the experiment. 523 students cleaned their teeth within ten minutes of eating. When possible, they used a toothbrush: when this was impossible, they washed their mouth thoroughly with water. The remaining 423 students merely cleaned their teeth when they went to bed and when they got up in the morning. All the students had their teeth X-rayed at the end of the first and second years. At the end of the first year, the night and morning group had three times as many decayed teeth as the clean-after-each-meal group. At the end of the second year, the latter group had 53 percent fewer decayed teeth than the former group. Dr. Faustick has cleaned his teeth after every meal for thirteen years and has not had a single decayed tooth. He pointed out that sugar is a major agent in dental decay, particularly the sugar in sweets, cakes, and soft drinks. Ideally you should keep a toothbrush in your pocket and use it immediately after you have finished eating. When this is impractical, you can at least make sure that you have a drink of water and let the water through your teeth to force out any particles of food. 7 out of 10 people lose at least half their teeth by the time they are fifty. Many have a complete set of false teeth by that time. In any case, neither toothache nor a visit to a dentist is very pleasant. So it is worthwhile making an effort to keep your own teeth as long as possible. The main preventative agent is simply water.
19. According to the passage, what type of food or drink is most likely to cause dental decay?
20. What does the passage tell us about the condition of Dr. Faustick’s teeth?
21. What does Dr. Faustick suggest to prevent dental decay?