阅读理解。 Most American kids love Halloween treats, but a bucket of Halloween candy can be a dentist's nightmare. Some parents try to get rid of half of the candy after their children go to bed, but dentists say parents also need to separate the good kinds of treats from the bad. It is not exactly what a child eats that truly matters, but how much time it stays in his mouth. According to pediatric dentists Dr Kaneta Lott, the most damaging stuff is something that is sticky or very hard and thus stays in the mouth for a long time. This is because we all have bacteria in our mouths. When we eat, the bacteria take our food as their food and produce an acid that destroys the surface of the teeth, causing cavities to form. The longer the food stays in the mouth, the more likely cavities will develop. Therefore, potato chips are worse than candy because they get stuck between teeth. For the same reason, raisins and crackers are not the best choice. Hard candies take a long time to consume and are also a bad choice for Halloween treats. If children really love candy, dentists recommend that they eat chocolate instead. Unlike hard candies, chocolate dissolves quickly in the mouth. Besides, chocolate contains tannins, which help to kill some of the bacteria in the mouth. But no matter what a child eats, brushing after each meal is still the best way to fight cavities. 1. What is the main purpose of this passage? [     ] A. To discuss how cavities can be treated. B. To point out the problems with Halloween celebrations. C. To tell parents what sweets are less damaging to their children's teeth. D. To teach parents the meaning of Halloween candies for their children. 2. Why are hard candies especially bad for teeth? [     ] A. They may break the child's teeth. B. They help bacteria to produce tannins. C. They contain too much sugar. D. They stay in the mouth for a long time. 3. According to the passage, which of the following is a better choice for Halloween treats? [     ] A. Chocolate B. Crackers C. Raisins D. Potato chips 4. According to the passage, which of the following is true of tannins? [     ] A. They are produced when the bacteria digest the food. B. They help to get rid of some bacteria in the mouth. C. They help chocolate to dissolve more quickly. D. They destroy the surface of the teeth.