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

Urbanisation occurs in many parts of the world, both in developed and developing nations. It refers to the process in which urban areas grow as people leave the countryside or more factories and towns are built in rural areas. This can lead to the concentration of populations into towns and cities. When properly planned, urbanisation can bring a number of benefits, especially for the economy and society. However, if it is poorly managed, it can pose a significant challenge to maintaining the sustainability of these areas.

Big cities attract a lot of people as they usually offer more employment opportunities as well as a better life. The economy of urban areas tends to grow rapidly as more businesses and factories are created, and more workers are needed. Economic growth also leads to an increase in household income and spending. This also encourages governments to spend money on better schools, hospitals, roads, public transport, and more. As cities expand, many more facilities such as public parks and gardens, swimming pools, libraries, cinemas, and sports centres are built. These are just some of the spaces that make city life exciting and appealing to people of all ages.

However, when too many people move to a big city, without good planning and investment, its infrastructure can become ineffective. For example, big urban areas can easily get overcrowded as railway networks and roads can't cope with the large number of passengers. The economic growth and expansion of big cities come at a big price for local residents as house prices and rents go up, and they can't afford to either buy or rent. Another problem is that big cities are experiencing more air pollution than rural areas. This can increase the risk of infections, heart disease, and lung cancer, and severely affect people who are ill.

Urbanisation is a process that cannot be stopped and the best way to minimise the problems caused by it is to plan for the growth and all the resources needed, and make sure the infrastructure can support the increasing population.

The word “they” in paragraph 3 refers to ______.