It is very important to have healthy teeth. Good teeth help us to chew our food .They also help us to look nice.
How does a tooth go bad? The 51begins in a little crack in the enamel(珐琅) covering of the tooth. This happens after germs and bits of food have__52 there. Then the decay slowly spreads inside the tooth. In the end, poison goes into the blood, and we may feel quite ill.
How can we keep our teeth 53 ? First, we ought to visit our dentist twice a year. He can fill the small holes in our teeth before they destroy the teeth. He can 53 54 our teeth to check that they are growing in the right way. 55 , many people wait until they have toothache before they see a dentist. Secondly, we should brush our teeth with a toothbrush and fluoride(氟化物) toothpaste at least__56 a day—once after breakfast and once before we go to bed. We can also use wooden toothpicks to 57between our teeth after a meal. Thirdly, we should eat food that is 58 to our teeth and our body: milk, cheese, fish, brown bread, potatoes, red rice, raw vegetables and fresh fruit. Chocolate, sweets, biscuits and cakes are bad, especially 59 we eat them between meals. They are harmful because they 60 our teeth and cause decay.
51. A. destruction B. decay C. fault D. hurt
52. A. stored B. collected C. laid D. piled
53. A. healthy B. plete C. effective D. painless
54. A. observe B. skim C. watch D. examine
55. A. Unexpectedly B. Certainly small C.Unfortunately D. Generally
56. A. once B. twice C. three times D. four times
57. A. drill B. dig C. clean D. explore
58. A. delicious B. valuable C. beneficial D. meaningful
59. A. what B. how C. whether D. when
60. A. stick to B. attach to C. fix on D. keep to 51---60 BBADC BCCDA
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(答案→)51---60 BBADC BCCDA
解析：
51.decay的意思是“腐烂”。(牙齿)的腐烂开始于覆盖在牙齿上的珐琅层上的小裂纹。destruction的意思是“破坏”；fault的意思是“缺点”；hurt的意思是“受伤”。答案为B。
52.collected在此为不及物动词，意思是“聚集；堆积”。细菌和食物聚集在牙缝后，就会引起牙齿珐琅层的破裂。store作不及物动词的意思是“贮藏”；laid是及物动词，意思是“放”；pile常和up连用，意思是“堆积”。答案为B。
53.healthy“健康的”，本篇短文讨论的是保持牙齿健康的问题。B为“完全的”；C为“有效的”；D为“无痛苦的”，均不合文意。答案为A。
54.“examine”表示医生检查身体(某一神经)，看牙医的目的是让他检查我们的牙齿是否生长正常。observe的意思是“观察”；skim的意思是“掠过”；watch的意思是“注视”。答案为D。
55. 每年至少看两次牙医可以保证我们牙齿的健康。他们可以为我们补牙，检查我们的牙龄是否生长正常。但是，不幸的是人们只在牙痛时才去看牙医。而A表示“出乎意料地”；B表示“肯定小”；D表示“一般地”，均不合题意。答案为C。
56.从后句once after breakfast and once before we go to bed可知。答案为B。
57.clean为“使……干净”。饭后使用牙签的目的是清除牙齿之间的食物。drill的意思是“钻”；dig的意思是“挖”；explore的意思是“探险”。答案为C。
58.beneficial的意思是“有益的”。为了保持我们牙齿的健康，我们必须吃有益于牙齿健康的食物。delicious的意思是“好吃的；美味的”；valuable的意思是“有价值的”；meaningful的意思是“意味深长的”。答案为C。
59.巧克力、糖果、饼干和蛋糕对牙齿有害，特别是我们在两顿饭之间吃这些东西。注意这四个选项的语法功能：what, how和whether只能引导名词性从句；而when既可以引导名词性从句，也可以引导时间状语从句。从句子的结构和意思都可判断出是时间状语从句，而不会是名词性从句。答案为D。
60.stick to的意思是“粘住”。巧克力、糖果、饼干和蛋糕会粘在我们的牙齿上，造成牙齿破裂，所以它们是有害的。attach to的意思是“附属于”；fix on的意思是“固定在”；keep一般不和介词to连用。答案为A。