Read the following passage and mark the letter A, B, C, or D to indicate the best answer to each of the following questions from 18 to 27.

Human populations have tended to increase over time. As more people were born, small groups of individuals found reasons to come together to form groups and, with the advent of agriculture, small sedentary communities. A small number of these settlements grew into what we now call cities. This kind of growth often corresponds with a shift from one way of organizing labor to another.

The world population has grown significantly and our economies have become more industrialized over the past few hundred years. As a result, many more people have moved into cities. This process is known as urbanization. Even after cities emerged, however, a large majority of people lived and worked in rural areas. It was not until large-scale industrialization began in the eighteenth century that cities really began to boom. Nearly half of all people now live in urban areas. They are attracted by jobs in manufacturing and the professions, as well as by increased opportunities for education and entertainment.

Urbanization is often discussed in reference to countries that are currently in the process of industrializing and urbanizing, but all industrialized nations have experienced urbanization at some point in their history. Moreover, urbanization is on the rise all over the globe.

[I] One effect of this huge increase in people living in urban areas is the rise of the megacity, which is a city that has more than 10 million inhabitants. [II] There are now cities with even more than that. [III] Another effect of urbanization is urban sprawl. [IV] Urban sprawl is when the population of a city becomes dispersed over an increasingly large geographical area. This movement from higher density urban cores to lower density suburbs means that as cities expand, they often begin to take up significant tracts of land formerly used for agriculture. Sprawl also increases the need for travel infrastructure, such as roads, because people’s homes are likely to be farther away from where they work and the amenities they enjoy.

As we move forward in the 21st century, the global population is likely to continue growing. Urban areas will continue to grow with the population. This continual growth presents complex challenges as we prepare for the cities of the future. How we choose to manage urbanization will have consequences for our world for many years to come.

The word “They” in paragraph 4 refers to _______. A. higher density B. suburbs C. cities D. movement