沉淀核酸时使用异丙醇与使用乙醇相比，最显著的优点是（） A.沉淀速度快 B.可增大溶

氟利昂是一种常见的制冷剂．氟利昂在紫外线的照射下能分解出氯原子破坏臭氧层，其破坏臭氧的循环示意图如下：

（1）请写出臭氧的化学式_______；臭氧与氧气化学性质不同的原因是________；

（2）经过①②③的反应后，氯原子的个数_______（填“减少”或“增加”或“不变”），反应①中的氯原子与反应③中的氯原子化学性质______（填“相同”或“不同”）；

（3）有科学家指出：“少量的氟利昂就能臭氧层产生巨大的影响”，请你根据循环示意图说明理由_______．

在Excel97中，可通过“格式”菜单中的列命令来调整行高。

阅读理解。

     American cities are similar to other cities around the world. In every country, cities reflect the values

of the culture. American cities are changing, just as American society is changing.

     After World War Ⅱ, the population of most large American cities decreased; however, the population

in many Sun Belt cities increased. Los Angles and Houston are cities where population shifts (转移) to

and from the city reflect the changing values of American society. In the late 1940s and early 1950s, city

residents became wealthier. They had more children so they needed more space. They moved out of their

apartments in the city to buy their own homes. They bought houses in the suburbs.

     Now things are changing. The children of the people who left the cities in the 1950s are now adults.

Many, unlike their parents, want to live in the cities. They continue to move to Sun Belt cities and older

ones of the Northeast and Midwest. Many young professionals are moving back into the city. They prefer

the city to the suburbs because their jobs are there; or they just enjoy the excitement and possibilities that

the city offers.

     This population shift is bringing problems as well as benefits. Countless poor people must leave their

apartments in the city because the owners want to sell the buildings or make apartments for sale instead

of for rent. In the 1950s, many poor people did not have enough money to move to the suburbs; now many

of these same people do not have enough money to stay in the cities.

     Only a few years ago, people thought that the older American cities were dying. Some city residents

now see a bright, new future. Others see only problems and conflicts. One thing is sure: many dying cities

are alive again.

1. What does the author think of cities all over the world?

A. They are similar.

B. They are hopeless.

C. They are alive.

D. They are different.

2. Why did American city residents want to live in the suburbs after World War Ⅱ?

A. Because older American cities were dying.

B. Because they were richer and needed more space.

C. Because they could hardly afford to live in the city.

D. Because cities contained the worst parts of society.

3. According to the 4th paragraph, a great many poor people in American cities ________.

A. are forced to move to the suburbs

B. are faced with housing problems

C. want to sell their buildings

D. need more money for daily expenses

4. We can conclude from the text that ________.

A. American cities are changing for the worse

B. the population is decreasing in older American cities

C. many people are now moving from American cities

D. people have different views on American cities

有以下程序：
#include＜stdio.h＞
main()
FILE *fp； int i,k=0,n=0;
fp=fopen("d1.dat","w");
for(i=1;i＜4;i++) fprintf(fp,"%d",i);
fclose(fp);
fp=fopen("d1.dat","r");
fscanf(fp,"%d%d",&k,&n); printf("%d %d\n", k,n);
fclose(fp);

执行后输出结果是( )。

A．1 2

B．123 0

C．1 23

D．0 0

下列选项中不是《离骚》一文特点的是（）。

A.明快清丽

B.想象丰富

C.情感炽烈

D.语言瑰丽