Question 24 is based on the passage you have just heard.

    ​​​​​​​Take care of your teeth and your teeth will take care of you. (22) Your teeth are a living part of your body. They have nerves and blood vessels. Diseased teeth can cause pain, die and fall out. Plaque is the main enemy of healthy teeth. Everyone has plaque. (23) It is a sticky colorless film that coats the teeth. Plaque is always forming on the teeth, especially at the gum line. If plaque is not removed, it builds up and gets under the gum line. Plaque that is left on the teeth for some time hardens. The result is tooth decay and gum disease. (24) The bacteria in plaque live on sugar. They change sugar into acids, which break down the tooth’s harder outer covering. If left untreated, decay goes deeper and deeper into the tooth. After a while, the decay reaches the nerves and blood vessels of the inner tooth. By the time this happens, the tooth has probably started to ache. (25) In addition to tooth decay, there are also gum diseases to watch out for. The bacteria in plaque make poisons that attack the gums. Small pockets form around the teeth. The pockets trap more bacteria and food particles. Finally, the bone supporting the teeth is attacked and starts to shrink. Teeth become loose and may fall out. Adults lose most teeth this way. Keep your mouth healthy. When you brush your teeth, do a good job.

(24). Why is sugar harmful to teeth?

解析：A。本题为细节题。问题为：为什么糖对牙齿有害？根据原文“The bacteria in plaque live on sugar. They change sugar into acids, which break down the tooth’s harder outer covering.”可知糖转化成酸以后，会破坏牙齿表面的更坚硬的覆盖物，故正确答案选A。