无效合同是指不具有经济约束力的合同。

Choose the best answer. 根据短文，选择最恰当的答案。

     Many people go to school for an education. They learn languages, history, politics, geography, physics,

chemistry and mathematics. Others go to school to learn a skill so that they can make a living. School education

is very important and useful. Yet, no one can learn everything from school. A teacher, no matter how much he

knows, can not teach his students everything they want to know. The teacher's job is to show his students how

to learn. He teaches them how to read and how to think. So, much more is to be learned outside school by the

students themselves.

     It is always more important to know how to study by oneself than to memorize some facts or a formula. It

is actually quite easy to learn a certain fact in history or a formula in mathematics. But it is very difficult to use

a formula in working out a math problem. Great scientists before us didn't get everything from school. Their

teachers only showed them the way. Edison didn't even finish primary school. But they were all so successful.

They invented so many things for mankind. The reason for their success is that they knew how to study. They

read books that were not taught at school. They would ask many questions as they read. They did thousands of

experiments. They worked hard all their lives, wasting not a single moment. Above all, they knew how to use

their brain.

1. People go to school to _______. [ ]

A. learn several subjects

B. make a living

C. get an education

D. learn from teachers

2. According to the passage, the most important thing a teacher should do is _______. [ ]

A. to teacher his students everything he knows

B. to know everything

C. to teach the students how to think

D. to teach the students how to study by themselves

3. To work out a math problem, you need to know ________. [ ]

A. only a certain formula

B. more than one formula

C. some facts

D. the method to solve it

4. Why were many scientists so successful? [ ]

A. They received good education.

B. They were very clever.

C. They knew how to learn.

D. They learned lots of facts and formulas.

5. How did great scientists study? [ ]

A. They read a lot of books and asked many questions while reading.

B. They did thousands of experiments.

C. They always worked hard and never wasted time.

D. All of the above.

下图为形态结构、生活习性互不相同的几种动物，请分析回答问题：

（1）若以“发育方式”为分类依据，将图中动物分成两组，C.D归为一组的依据是                               。

（2）D的发育过程经过的时期有                                         ，属于          发育。

（3）青蛙的幼体生活在水中，用　  　呼吸；青蛙的生殖特点是：　　                      　；因此，青蛙的生殖和发育都离不开水。

患者，男，72岁。戴用全口义齿。在做侧方咬合时，平衡侧基托翘动、脱落，其可能原因是（）

A.正中牙合不平衡

B.侧方牙合不平衡

C.前伸牙合不平衡

D.垂直距离过高

E.义齿固位差

工人提前退休将降低平均劳动生产率。

写出下列处方中各成分的作用

渗透压调节剂

A．醋酸氢化可的松微晶25g
B．氯化钠8g
C．吐温80 3.5g
D．羧甲基纤维素钠5g
E．硫柳汞0.01g