Unit 9 covers 1900 to the present, and its central move is evaluative rather than narrative: almost every development it names produced both a benefit and a cost, and questions reward answers that hold both.

The CED names the advances directly:

• New modes of communication 通信方式 - radio, cellular technology, and the internet.
• Energy technologies 能源技术, including the use of petroleum 石油 and nuclear power 核能.
• Medical innovations 医学创新, including vaccines 疫苗 and antibiotics 抗生素, which increased lifespans 寿命.
• The Green Revolution 绿色革命 and commercial agriculture, which increased the global food supply.
• More effective forms of birth control 节育, which gave women greater control over fertility 生育, transformed reproductive practices, and contributed to declining fertility rates in industrialised states.

Note the last one carefully. It is a technological change with a demographic and a social consequence, which makes it unusually useful evidence in questions about the changing role of women.

Diseases, as well as medical and scientific developments, had significant effects on populations across the century. The CED splits the effect in two:

• Diseases associated with poverty persisted, while other diseases emerged as epidemics and pandemics.
• Diseases associated with longer lifespans - conditions such as heart disease and neurological illnesses of old age - became more prominent precisely because medical advances extended life.

That second point is the unit's logic in miniature: the success of one intervention produced the conditions for a different problem. It is not a contradiction, and saying so is what makes an answer analytical.

Human activity contributed to deforestation 森林砍伐, desertification 荒漠化, a decline in air quality 空气质量下降, and increased consumption of the world's supply of fresh water 淡水.

The release of greenhouse gases 温室气体 and pollutants into the atmosphere provoked debates about climate change 气候变化. The CED's word is debates - the examinable content is that the science and its policy implications became a subject of international argument, not a settled instruction.

Three developments define the period's economic history, and all three accelerated after 1990:

• Many governments encouraged free-market economic policies 自由市场政策, a trend accelerated by the end of the Cold War.
• Revolutions in information and communications technology led to the growth of knowledge economies 知识经济 in some regions, while other regions remained centred on agriculture or manufacturing.
• Changing economic institutions, multinational corporations, and regional trade agreements 区域贸易协定 reshaped how production and trade were organised across borders.

The continuity worth naming: this is the same story as Unit 6's economic imperialism, in new institutional form. Firms in wealthy states again hold disproportionate influence over the terms on which producers elsewhere participate.

Rights-based discourses 权利话语 challenged old assumptions about race, class, gender, and religion. In much of the world, access to education 受教育机会 and participation in new political and professional roles became more inclusive - an uneven process, but a real one.

At the same time, movements throughout the world protested the inequality of the environmental and economic consequences of global integration 全球一体化. Both halves are examinable: greater inclusion, and organised objection to how the benefits were distributed.

The CED makes three separate claims about culture, and they are not the same claim:

• Political and social changes of the twentieth century led to changes in the arts and other forms of cultural expression.
• Arts, entertainment, and popular culture increasingly reflected global influence 全球影响 - forms borrowed, adapted, and recombined across regions.
• Consumer culture 消费文化 became globalised and transcended national borders.

The third is the strongest for essays because it is the most measurable: the same products, brands, and consumption patterns appearing across very different societies.

Responses to rising cultural and economic globalisation took a variety of forms. The CED deliberately leaves the category broad, and a good answer fills it with distinct kinds rather than repeating one:

• Movements defending local or national cultural identity against homogenising influence.
• Organised opposition to trade agreements and international financial institutions on economic grounds.
• Religious and traditionalist movements 宗教与传统主义运动 rejecting the social changes that accompanied integration.

New international organisations 国际组织, including the United Nations 联合国, emerged to manage problems no single state could handle alone - security, health, refugees, trade, and the environment.

This closes the arc of the course: Unit 7 opened with the collapse of an order and unprecedented conflict; Unit 9 ends with the institutions built in response, and with the argument - still unresolved - about how much authority they should hold.

The unit's reasoning skill is continuity and change, applied across the whole modern period.

Worked example (a real AP exam question). AP World short-answer questions here ask, for example, to "Explain one way in which technological developments after 1900 affected the world's population." A full-mark answer names the development and traces its effect: "Medical innovations such as vaccines and antibiotics reduced deaths from infectious disease, which increased average lifespans and contributed to rapid population growth, particularly in regions where these treatments were introduced faster than birth rates fell." Notice the mechanism: a reason the population changed, not the statement that it did.

• Unit 9 questions are evaluative. For almost every advance, name the benefit and the cost. That single habit lifts most answers here.
• Birth control is technological, demographic, and social evidence at once - unusually flexible in questions about women's changing roles.
• The rise of diseases of longer life is caused by medical success. Say that explicitly; it demonstrates causal reasoning.
• On climate, the CED's word is debates. Write about the argument, not a verdict.
• Connect the global economy back to Unit 6: the same asymmetry, new institutions. Cross-period links score well on essays.
• For resistance to globalisation, give distinct kinds - cultural, economic, religious - not one kind three times.