已知x+y=3 ，则7-2x-2y 的值为；

关于动物的先天性行为和后天学习行为的对比中正确的一项是（　　）

A．后天学习行为比先天性行为更为高等重要

B．后天学习行为只存在于高等动物中

C．先天性行为和后天学习行为都需要一定的遗传基础

D．先天性行为和后天学习行为都是对环境的各种刺激做出的反应

公路货运工作量调查的内容是什么？

任何两个并发进程之间( )。

A．一定存在互斥关系

B．一定存在同步关系

C．资源静态分配策略

D．可能存在同步或互斥关系

阅读理解。

     Grown-ups know that people and objects are solid. At the movies, we know that if we reach out to

touch Tom Cruise, all we will feel is air. But does a baby have this understanding?

     To see whether babies know objects are solid, T.Bower designed a method for projecting an optical

illusion (视觉影像) of a hanging ball. His plan was to first give babies a real ball, one they could reach

out and touch, and then to show them the illusion. If they knew that objects are solid, and they show

surprise in their faces and reach out for the illusion and found empty air, they could be expected

movements. All the 16-to 24-week-old babies tested were surprised when they reached for the illusion

and found that the ball was not there.

     Grown-ups also have a sense of object permanence. We know that if we put a box in a room and

lock the door, the box will still be there when we come back. But does a baby realize that a ball that rolls

under a chair does not disappear or go to never-never land?

     Experiments done by bower suggest that babies develop a sense of object permanence when they are

about 18 weeks old. In his experiments, Bower used a toy train that went behind a screen. When

16-week-old and 22-week-old babies watched the toy train disappear behind the left side of the screen,

they looked to the right, expecting it to reappear. If the experimenter took the train off the table and lifted

the screen, all the babies seemed surprised not to see the train. This seems to show that all the babies had a sense of object permanence. But the second part of the experiment showed that this was not really the

case. The researcher substituted (替换) a ball for the train when it went behind the screen.

The 22-week-old babies seemed surprised and looked back to the left side for the train. But

16-week-old babies did not seem to notice the switch (更换). Thus, the 16-week-old babies seemed to

have a sense of "something permanence," while the 22-week-old babies had a sense of object

permanence related to a particular object.

 1. The passage is mainly about ______.

A. babies' sense of sight    

B. effects of experiments on babies

C. babies' understanding of objects    

D. different tests on babies' feelings

2. In Paragraph 3, "object permanence" means that when out of sight, an object ______.

A. still exists  

B. keeps its shape  

C. still stays solid  

D. is beyond reach

3. What did Bower use in his experiment?

A. A chair    

B. A screen        

C. A film      

D. A box

4. Which of the following statements is true?

A. The babies didn't have a sense of direction.

B. The older babies preferred toy trains to balls

C. The younger babies liked looking for missing objects.

D. The babies couldn't tell a ball from its optical illusion.

抓斗闭合时最大间隙处的长度不应大于（）mm.

A、200

B、250

C、300