已知一列平面简谐波沿Ox轴正向传播，波速为u=400m·s－1，频率v=20Hz，t=0时刻波形曲线如图所示，则波动表达式为（

Area and Population of ContinentsDirections: For this part, you are allowed 30 minutes to write a composition of no less than 120 words based on the following chart. Interpret these charts and discuss how the information in the Area chart is related to information in the Population chart. Explain the conclusions you have reached from the information in the two charts. Be sure the charts support your conclusions.

AREA POPULATION

Asia 30 % Asia 58%

Africa 20% Europe 16%

North America 16% Africa 11%

South America 12% North America 9%

Antarctica 9% South America 5%

Europe 7% Oceania 1%

Oceania 6% Antarctica 0%

a: Oceania a: Oceania

b: Europe b: South America

c: Antarctica c: North America

d: South America d: Africa

e: North America e: Europe

f: Africa f: Asia

g: Asia

A．The percentage is high.

B．The percentage is low.

C．The amount of the population is not so low.

D．It is not mentioned in the passage.

A．The percentage is high.

B．The percentage is low.

C．The amount of the population is not so low.

D．It is not mentioned in the passage.

A large percentage of the population in this country is ______.

A．illogical

B．illegimate

C．illiterate

D．illegible

CROWDING THE PLANET

This chart shows how much the world's population grew between 1900 and 2000, and how much experts believe it will grow by 2025. The numbers are in billions.

What was the world population in 1950？

A．Half a billion

B．2 billion

C．2.5 billion

D．5 billion

Chinese ________ the largest percentage of population in Hong Kong.

A．make up

B．take up

C．hold up

D．turn up

What percentage of the population should pay their tuition？

A．50.

B．2.

C．4.

D．6.

What is the percentage of the population who are illiterate？

A．6%.

B．5%.

C．2%.

D．8%.

cent. Until modem times, urban settlements attracted only a small percentage of the total population and rarely reached a population of more than a few thousand inhabitants.

In the past two centuries, the world has experienced rapid urbanization. Relatively developed countries have been transformed from predominantly rural to predominantly urban societies. The process of urbanization involves two changes in the distribution of population in a country: an increase in the number of people living in urban settlements and an increase in the percentage of people living in urban settlements. These two types of increases occur for different reasons.

The increase in the number of urban residents is a function of the total population increase in the society. As the population of the country grows, some of those additional people inevitably will live in urban settlements.

Rome, the largest urban settlement of the ancient world, probably was no larger in population than the modern Des Moines, Iowa. Most urban historians estimate that the first settlement to exceed 1 million inhabitants was London in approximately 1810. Today, more than 100 cities exceed 2 million.

At the same time, urbanization means an increase in the percentage of urban dwellers and a corresponding decrease in the percentage of rural residents. The increase in the percentage of urban residents is a function of the society's changing economic structure. A high percentage of urban residents reflects the fact that most of the jobs are in factories, offices, and services rather than on farms.

In 1800, only 3 percent of the world's population lived in urban settlements, compared with more than 40 percent today. If current trends continue, by the end of the twentieth century, the population of urban settlements may exceed that of rural settlements for the first time in human history.

The increase in the percentage of urban residents is related to ______.

A．total population increase in society

B．the society's changing economic structure

C．the extent of social development

D．the extent of urbanization