①机身蒙皮使用的是第三代铝锂合金材料．下列不属于铝锂合金性质的是____（填字母）．

a．密度大       b．硬度大       c．抗腐蚀

②飞机轮胎属于橡胶制品．下列材料可用来合成橡胶的是____（填字母）．

a．石英砂   b．石灰石     c．异戊二烯

③机体先进复合材料的使用率达12%．下列属于复合材料的是____（填字母）．

a．钛合金            b．玻璃钢                c．普通陶瓷．

冬天里，户外装有水的水缸常会出现破裂，这是因为水缸里的水结成冰后，其体积____ ，密度____ （两个选填“变大”、“变小”或“不变”）

    American people like to say "Thank you " or say something kind when others help them．People of many countries do so，too．It is a very good habit．
    You should say"Thank you'when someone passes you the salt on the table，when someone walking ahead of you keeps the door open for you and when someone says you always do your work well or you have a nice thing or your city is very beautiful．“Thank you”is used not only between friends，but also between parents and children，brothers and sisters．
    “Excuse me”is another short sentence they use．When you hear someone says“Excuse me”behind you，you know that someone wants to walk past you without touching you．It’s not polite to break(打断)others when they are talking．If you want to speak to one of them。say‘Excuse me’first，and then begin talking．You should also do so when you begin to cough or make any noise before others．
Let‘s learn to say‘Thank you’and‘Excuse me’．