听力原文： How many teeth have you had failed in the past two years? If you follow the advice of Dr Forsdic, you may be able to reduce the number of your visits to dentist. Dr Forsdic 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 10 minutes of eating. When possible, they used the tooth- brush. 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 3 times as many decayed teeth as the clean-after-each- meal group. At the end of the second year, the latter group had 53% fewer decayed teeth than the former group. Dr Forsdic has cleaned his teeth after every meal for 13 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. Seven out of ten people lose at least half their teeth by the time they are 50. 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. According to the passage ,what type of food/drink is most likely to cause dental decay?