简答机油系统循环流动的机油的主要作用。
参考答案：
（1）润滑作用；（2）冷却作用；（3）清洗作用；（4）密封作用；（5）防锈作用。
根据实际情况或想象，回答下列问题。将答案写在题号下相应的横线上。
1. What are you going to do for your holiday?
2. Do you go for your holiday with your friends?
3. Where will you go for your holiday?
4. How many people are there in your group?
5. What do you think of your holiday?
属于边缘封闭区（）
A.上颌牙槽嵴顶
B.磨牙后垫
C.切牙乳突
D.颊侧前庭
E.颊系带
