Mother told Jim to_______the milk until it boiled and then turn off the gas.

A.observe         B.watch          C.see                D.examine

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 __51 begins 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__57 between 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. complete      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