3. twice a day.英语中的习惯说法。又如：一月叁次three times a month.

2. because.要回答前面why提出的问题，应用because.

1. open.张开嘴的“张开”应用open.在助动词will后直接用动词原形。

9、根据上下文和括号里的汉语提示，在下面的空白处写出正确的单词和短语，使短文意思完整。

　　 When you laugh， you will (1)______(张开) your mouth and your teeth. The healthier those teeth are， the happier you look. Why is that？

　　 It‘s (2)______(因为) your teeth are important in many ways. If you take care of them， they’ll help to take care of you. Strong， healthy teeth help you eat the right food to help you grow. They also help you speak clearly.

　　 You can take care of your teeth by doing like these：

　　 Brush your teeth (3)______(一日两次) after breakfast and before bedtime. If you can， brush (4)______(午饭后) or after eating sweet cakes.

　　 Brush all of your teeth， not just the front ones. Spend some time on the teeth along the sides and in the back.

　　 Take your time while brushing. Spend (5)______(至少) 3 minutes each time you brush.

　　 Be sure your toothbrush is soft(柔软的)。 Ask your parent to help you get a new toothbrush (6)______(每三个月)。

　　 Learn how to floss(用牙线清理) your teeth， which is a very important way to keep them healthy. It feels strange when you do it at first， but soon you‘ll (7)______(习惯于) doing it. The floss gets rid of food that’s hidden between your teeth.

　　 Brushing and flossing (8)______(保持) your teeth healthy. You also need to care about what you eat and drink. Eat (9)______(许多) fruits and vegetables and drink water (10)______ (代替) drinks.

　　 「答案与解析」

15. of.固定搭配：a lot of许多。

14. taught / learnt / learned.由in the classroom可知是老师“教给”或自己“学到”，注意是被动语态，要用过去分词。

13. know.第7空后有明显的提示：know how to study.

12. One.从后文的答语和谓语动词is可知，是其中的一个答案。one of“……中的一个”。

11. invent / do / make 由前面的inventors和后文的“改变世界”可知，他们发明了许多东西。

10. at / in / from.句意：发明家们并没有在学校或从学校学到了一切。