The word It in paragraph 3 refers to_________.

Câu hỏi: Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.
It is estimated that by 2050 more than two-thirds of the world's population will live in cities, up from about 54 percent today. While the many benefits of organized and efficient cities are well understood, we need to recognize that this rapid, often unplanned urbanization brings risks of profound social instability, risks to critical infrastructure, potential water crises and the potential for devastating spread of disease. These risks can only be further exacerbated as this unprecedented transition from rural to urban areas continues.
How effectively these risks can be addressed will increasingly be determined by how well cities are governed. The increased concentration of people, physical assets, infrastructure and economic activities mean that the risks materializing at the city level will have far greater potential to disrupt society than ever before.
Urbanization is by no means bad by itself. It brings important benefits for economic, cultural and societal development. Well managed cities are both efficient and effective, enabling economies of scale and network effects while reducing the impact on the climate of transportation. As such, an urban model can make economic activity more environmentally-friendly. Further, the proximity and diversity of people can spark innovation and create employment as exchanging ideas breeds new ideas.
But these utopian concepts are threatened by some of the factors driving rapid urbanization. For example, one of the main factors is rural-urban migration, driven by the prospect of greater employment opportunities and the hope of a better life in cities. But rapidly increasing population density can create severe problems, especially if planning efforts are not sufficient to cope with the influx of new inhabitants. The result may, in extreme cases, be widespread poverty. Estimates suggest that 40% of the world's urban expansion is taking place in slums, exacerbating socio-economic disparities and creating unsanitary conditions that facilitate the spread of disease.
The Global Risks 2015 Report looks at four areas that face particularly daunting challenges in the face of rapid and unplanned urbanization: infrastructure, health, climate change, and social instability. In each of these areas we find new risks that can best be managed or, in some cases, transferred through the mechanism of insurance.
The word It in paragraph 3 refers to_________.
A. Urbanization
B. Development
C. Population
D. Impact